ACHROMATIC  MICROSCOPES, 


OAKST.HDSF 


I 


r 


} 


Y 


‘V  / 


f 


r 


• -?P' 


+■ 


Digitized  by  the  Internet  Archive 
in  2017  with  funding  from 

University  of  liiinois  Urbana-Champaign  Aiternates 


https://archive.org/detaiis/iliustratedscien00jwgr_0 


ILLUSTRATED 


SCIENTIFIC  AND  DESCRIPTIVE 


CATALOGUE 


OF 


ACHEOMATIC  MICEOSCOPES, 


MANUFACTURED  BY 


J.  & W.  GRUNOW  & CO., 

NEW  HAVEN,  CONN. 

30  CE^NTTS. 

NEW  HAVEN: 

T.  J.  STAFFORD,  PRINTER. 


1857. 


Entered, 

According  to  Act  of  Congress,  in  the  year  1857, 

By  J.  & W.  GRUNOW  & CO., 

In  tlie  Clerk’s  Office  of  the  District  Court  of  Connecticut. 


PREFACE. 


In  presenting  to  the  public  this  Illustrated  and  Descriptive 
Catalogue,  it  is  our  intention  to  furnish,  especially  to  those 
living  at  a distance,  the  means  of  becoming  acquainted  with 
the  various  forms  and  qualities  of  the  microscopes  and  micro- 
scopical apparatus  which  we  manufacture. 

Having  been  engaged  almost  exclusively,  for  a series  of 
years,  in  the  manufacture  of  this  important  instrument,  we 
have  devoted  great  care  and  attention  to  improving  the  struc- 
ture of  the  mechanical  parts,  as  well  as  to  perfecting  the  qual- 
ity and  efficiency  of  our  object-glasses,  and  the  optical  parts 
in  general. 

The  extensive  patronage  with  which  we  have  been  favored 
affords  sufficient  evidence  that  in  our  efforts  to  deserve  the 
confidence  of  the  scientific  public  we  have  not  been  without 
success. 

But  while  it  has  been  our  object  to  bring  to  perfection  the 
branch  of  practical  optics  to  which  we  have  devoted  ourselves, 
we  have  been  constantly  adding  to  our  facilities  for  manufac- 
turing the  various  apparatus  connected  with  the  microscope. 
By  the  introduction  of  machinery  especially  adapted  to  our 
purposes,  and  by  a judicious  division  of  labor,  we  have  en- 
deavored to  secure  the  most  finished  workmanship  in  every 
part,  at  such  prices  as  shall  place  the  most  perfect  instruments 
^within  the  reach  of  all  the  lovers  of  science  and  students  of 
nature. 


IV 


PREFACE. 


We  are  daily  receiving  inquiries  from  persons  who  desire 
information  and  advice  in  regard  to  the  selection  of  micro- 
scopes adapted  to  their  particular  purposes.  It  frequently 
happens  that  persons  making  these  inquiries  have  had  but 
little  opportunity  to  become  acquainted  with  the  theory  and 
uses  of  microscopes  made  in  the  most  recent  and  improved 
style.  Properly  to  reply  to  all  these  inquiries  would  absorb 
too  much  of  our  time,  which  we  desire  to  devote  exclusively 
to  the  more  delicate  and  important  labor  of  our  art.  We 
have  therefore  decided  to  publish  in  our  catalogue  such  infor- 
mation as  shall  enable  every  one  to  understand  the  philosophy 
and  structure  of  the  most  improved  microscopes,  and  to  jndge 
of  the  qualities  of  different  microscopes  that  may  be  offered 
to  their  patronage. 

So  rapidly  has  the  microscope  been  improved  within  a very 
recent  period,  that  instruments  which  were  made  but  a few 
years  since  have  become  of  little  value,  compared  with  the 
improved  achromatic  miscroscopes  which  are  now  made  by 
the  best  opticians  of  England  and  America. 

This  rapid  advance  of  improvement  has  caused  a large  stock 
of  microscopes  of  inferior  quality  to  be  thrown  into  the  market 
at  very  low  prices,  and  those  who  are  not  aware  of  the  great  su- 
periority of  the  achromatic  microscope,  as  at  present  con- 
structed, are  induced  to  purchase  cheap,  but  inferior^  instru- 
ments. With  this  species  of  trade  it  is  not  our  purpose  to  com- 
pete. But  to  those  who  desire  to  obtain  microscopes  of  the  best 
construction,  combining  the  most  recent  scientific  improve- 
ments and  fitted  for  the  prosecution  of  the  highest  order  of  sci- 
entific inquiries,  we  offer  our  microscopes  with  confidence  that 
wherever  their  merits  are  known  they  will  give  ample  satisfac- 
tion to  purchasers. 

J.  & W.  Grunow  & Co. 

Kew  Haven,  Conn.,  Oct.  1,  1857. 


CONTENTS 


ACHROMATIC  MICROSCOPES. 

CHAPTEE  1. 


THEORY  OF  THE  MICROSCOPE. 

SECTION  PAGE 

1.  Introduction, 1 

2.  Simple  Lenses, 1 

3.  Foci  of  Lenses, 1 

4.  Aberration, 2 

5.  Spherical  Aberration, 2 

6.  Amount  of  Spherical  Aberration, 3 

7.  Proportionate  Curvature, 4 

8.  Negative  Aberration, 4 

9.  Aberration  of  Sphericity  : Curvature  of  Image, 4 

10.  Chromatic  Aberration, 5 

11.  First  method  of  diminishing  Chromatic  Aberration, 6 

12.  Second  “ “ “ “ “ 6 

13.  Achromatism, 6 

14.  Combined  Lenses, 7 

15.  Angular  Aperture, 8 

16.  Oblique  Illumination, 9 

17.  Lister’s  Discoveries, 9 

18.  Essential  requisites  of  good  Object-Glasses, 9 

19.  Mr.  Lister’s  two  Preliminary  Propositions, 10 

20.  Principles  discovered  by  Mr.  Lister, 10 

21.  Aplanatic  Object-Glasses, 11 

22.  Superiority  of  English  and  American  Object-Glasses, 12 

23.  Compound  Achromatic  Microscope, 12 

24.  Positive  Eye-piece, 12 

25.  Negative  Eye-piece, 12 

26.  General  view  of  Compound  Achromatic  Microscopes, 13 

27.  Enlarged  section  of  Achromatic  Microscope, 15 

28.  Action  of  Negative  Eye-piece, 16 

29.  Advantage  of  Over-correcting  Object-Glass, 17 

30.  Negative  Eye-piece  nearly  Achromatic, 17 

31.  Use  of  the  term  Achromatic  Objective, 18 

32.  Aberration  produced  by  Glass  Cover, 19 

33.  Objective  Corrected  for  Glass  Cover, 20 

34.  Eye-pieces  of  different  Powers, 21 


vi  CONTENTS. 


SECTION  PAGE 

35.  Draw-tube, 21 

36.  Names  applied  to  Object-Glasses, 21 

37.  Foci  of  Higher  Powers  inconveniently  near  to  Objective, 22 


CHAPTEE  IL 

MECHANICAL  POETION  OF  THE  MICROSCOPE. 

38.  Modern  Improvements, 23 

39.  Base, 23 

40.  Arrangement  for  Inclining  the  Instrument, 23 

41.  Stage, 24 

42.  Adjustment  of  Focus, 24 

43.  Simplicity  and  Facility  of  Adjustment, 24 

44.  Description  of  Microscope  Stands, 24 

45.  No.  1,  Educational  Microscope, 25 

46.  No.  2,  Student’s  Microscope, 27 

47.  No.  3,  Student’s  Microscope, 28 

48.  No,  4,  Student’s  Larger  Microscope, 29 

49.  Stage  movable  by  Back  and  Screw, 30 

50.  No,  5,  Another  form  of  Student’s  Microscope, 31 

51.  Chevalier’s  Prismatic  Body, 32 

52.  Prof,  Bailey’s  Indicator  Stage, 33 

53.  No,  6,  Portable  Microscope, 35 

54.  No.  7,  Large  Microscope, 37 

55.  Chemical  or  Inverted  Microscopes, 38 

56.  No.  8,  Simple  form  of  Inverted  Microscope, 39 

57.  More  complete  Inverted  Microscope, 40 

58.  Obiect-<Glasses, 40 

59.  Rules  for  Adjusting  Object-Glasses, 42 

60.  Delicacy  of  Adjustment  for  Thin  Covers, 42 

61.  Second  Class  Objectives, 43 


CHAPTEE  III. 

ACCESSORY  APPARATUS. 

62.  Micrometers, 46 

63.  Glass  Stage  Micrometers  Mounted  in  Brass, 46 

64.  The  Cobweb  Micrometer, 45 

65.  Ross’  Eye-piece  Micrometer, 47 

66.  Jackson’s  Micrometer, 47 

67.  Comparative  Merits  of  Micrometers, 48 


CONTENTS.  Vll 


SECTIOS 

68.  Dr.  White’s  Micrometer, 49 

69.  Prof.  J.  Lawrence  Smith’s  Goniometer  and  Micrometer, 60 

70.  Method  of  using  the  Goniometer, 51 

71.  Value  of  Lines  in  Eye-piece  Micrometers, 51 

72.  Method  of  using  the  Micrometer, 53 

73.  Fraunhofer’s  Stage-Screw  Micrometer, 54 

Camera  Cacida. 

74.  Wollaston’s  Camera  Lucida, 56 

75.  Nachet’s  Camera  Lucida, 56 

76.  Soemmering’s  Steel  Speculum, 57 

77.  Using  the  Camera, 57 

78.  Camera  Lucida  applied  to  Micrometry, 58 

■Tli§cellasaeou§  Apparatus. 

79.  Movable  Diaphragm  Plate, 58 

80.  Bull’s-Eye  Condenser, 59 

81.  Smaller  Bull’s-Eye  Condenser, 60 

82.  Achromatic  Condenser, 61 

83.  Nachet’s  Prism  for  Oblique  Illumination, 62 

84.  Lieberkuhn  Speculum, 63 

85.  Erector, 64 

86.  Orthoscopic  Eye-piece, 64 

87.  Compressor, 65 

88.  Animalcule  Cage  with  Screw, 65 

89.  Simple  Animalcule  Cage, 66 

90.  Stage  Forceps  : Hand  Forceps, 66 

91.  Frog  Plate, 66 

92.  Machine  for  cutting  circles  of  thin  glass, 67 

93.  Instrument  for  making  cells  of  gold  size, 69 


CHAPTER  lY. 

POLARIZED  LIGHT  AND  ITS  APPLICATION  TO  THE 
MICROSCOPE. 

94.  Theories  of  Light, 70 

95.  Double  Refraction  and  Polarized  Light, 70 

96.  Polarization  by  Reflection, 71 

97.  Polarization  by  Refraction, 72 

98.  Polarization  by  bundles  of  thin  plates, 74 

99.  Double  Refraction, 74 

100.  Double  Refraction  of  Iceland  Spar, 75 


Vlll 


CONTENTS. 


SECTION  PAGE 

101.  Polarization  produced  by  Double  Refraction, 77 

102.  Nicol’s  Single  Image  Prism, 77 

103.  Common  and  Polarized  Light  Contrasted, 79 

Theory  of  Polarized  L.iglit. 

104.  Undulatory  Theory, 79 

105.  Illustrations  of  wave  motion, 80 

106.  Polarization  illustrated  by  resultant  motion, 81 

107.  Polarizing  effect  of  Iceland  Spar, 82 

108.  Familiar  Illustrations, 83 

109.  Partial  Polarization, 83 

Polarized  I^iglit  applied  to  the  Microscope. 

110.  Polarizing  Apparatus, 85 

111.  Tourmaline  Plates, 86 

112.  Herapathite, 87 

113.  Value  of  Polarized  Light  in  Microscopic  Investigations, 87 

114.  Colored  Polarization, 87 

115.  Cause  of  Colors  produced  by  plates  of  selenite, 88 

116.  Method  of  Varying  Colors, 88 

117.  Selenite  Stage, 90 

118.  Polarizer  with  Revolving  Selentine  Cai-rier, 90 

119.  Delicate  structures  viewed  by  colored  polarized  light, 91 

120.  List  of  Objects  for  Polariscope, 92 

CIIAPTEK  Y. 

PRACTICAL  DIRECTIONS. 

121.  Care  of  the  Microscope, 94 

122.  Illumination, 95 

123.  Choice  of  a Microscope, 95 

124.  Qualities  of  Object-Glasses, 96 

1.  Defining  power, 96 

2.  Resolving  power, 97 

Method  of  measuring  angular  aperture, 97 

3.  Flatness  of  field, 97 

4.  Depth  or  Extent  of  Definition, 97 

PRICE  LIST, 99 


ACHROMATIC  MICROSCOPES. 


CHAPTER  I. 

THEORY  OF  THE  MICROSCOPE. 

1.  Introduction.  In  calling  public  attention  to  the  claims 
of  Compound  Achromatic  Microscopes,  and  their  great  superi- 
ority over  all  others,  it  is  necessary  to  explain  the  defects  of 
simple  lenses,  the  general  structure  and  action  of  achromatic 
lenses,  and  also  the  entire  optical  and  mechanical  arrangement 
of  the  most  improved  Compound  Achromatic  Microscopes.  It  is 
presumed  that  every  reader  of  this  catalogue  understands,  or 
can  easily  learn  from  ordinary  works,  the  simple  elements  of 
optics,  and  the  structure  of  simple  and  compound  miscroscopes 
formed  of  simple  lenses. 

2.  Simple  liCiises.  The  names  applied  to  the  various  forms 
of  simple  lenses  sufficiently  explain  their  structure,  and  are 
generally  understood.  All  lenses  are  supposed  to  be  bounded 
by  plane,  or  spherical,  surfaces  ; for  although  some  other  forms 
would  be  desirable  if  they  could  be  made  with  sufficient  accu- 
racy, it  is  found  in  practice  that  only  plane  and  spherical  sur- 
faces can  be  wrought  so  perfectly  as  to  render  them  available 
for  optical  purposes. 

3.  Foci  of  Fenses.  A piano  convex  lens  E E,  Fig.  1, 
(or  any  spherical  lens  thicker  in  the  centre  than  at  its  edges,) 
refracts  parallel  rays  of  light  L L,  to  a point  F,  called  its 

1 


2 


THEOEY  OF  THE  MICROSCOPE. 


prineipal  focus.  If  a pencil  of  light  diverging  from  a point 
behind  the  lens,  as  F',  more  distant  from  the  lens  than  the 

Fig.  1. 


principal  focus,  falls  upon  the  lens,  it  will  be  refracted  to  a focus 
f,  also  more  distant  than  the  principal  focus.  When  F'  ap- 
proaches the  lens,  f will  recede  from  it,  and  vice  versa.,  hence 
these  two  are  called  the  conjugate  foci  of  the  lens.  The  rela- 
tive distances  of  the  conjugate  foci  have  a certain  relation  to 
the  distance  of  the  principal  focus  F, 

4.  Aberration.  In  a perfect  lens  the  above  statements 
would  be  strictly  correct,  but  in  all  spherical  lenses  it  is  found, 
on  careful  examination,  that  the  rays  falling  upon  the  lens  at 
different  distances  from  the  centre,  do  not  all  meet  in  a single 
point,  but  are  subject  to  two  different  causes  of  error,  called 
spherical  and  chromatic  aberration,  which  will  now  be  ex- 
plained. 

Fig.  2. 


5.  Spherical  Aberration.  Let  F,  Fig.  2,  represent  tlie 
focus  of  two  parallel  rays  L L,  near  the  centre  of  the  lens  ; 
then  if  1 1 represent  the  rays  falling  upon  the  borders  of  the 
lens,  they  will  be  so  refracted  as  to  meet  at  a point  f,  con- 


J.  & W.  GRUNOW  & GO’S  ILLUSTRATED 


SPHERICAL  ABERRATION. 


3 


siderably  nearer  to  the  lens  than  the  point  F,  where  the  central 
parts  of  the  pencil  converge.  The  distance  F f,  is  called  the 
longitudinal  spherical  aberration,  and  a the  smallest  diam- 
eter of  the  converging  pencil,  is  called  the  lateral  spherical 
aberration, 

6.  It  is  easily  seen  that  if  the  piano  convex  lens  is  turned 
with  its  convex  surface  towards  parallel  rays,  the  points  where 
the  lateral  rays  undergo  refraction  would  then  be  in  advance 
of  the  positions  where  the  rays,  passing  near  the  centre,  are 
refracted,  and  consequently  the  spherical  aberration  would  be 
diminished  by  the  difference  between  the  thickness  of  the  lens 
at  the  centre  and  the  thickness  at  the  border.  The  difference  is 
really  much  greater  than  this,  for  if  the  plane  side  of  the  lens 
is  turned  towards  parallel  rays,  as  shown  in  Fig.  2,  the  longi- 
tudinal spherical  aberration  is  about  times  the  thickness  of 
the  lens,  but  if  the  opposite  face  is  towards  parallel  rays,  the 
aberration  is  only  about  lyVo  times  its  thickness.* 

A double  convex  lens  has  the  least  spherical  aberration 
when  the  two  faces  have  their  radii  of  curvature  in  the  propor- 
tions of  about  one  to  six.  Such  a lens  with  its  flatter  surface 

* The  spherical  aberration  of  a lens  varies  with  the  refractive  power  of  the 
material  of  which  the  lens  is  made. 

The  proportions  here  given  are  taken  from  Lardner’s  Optics.  Other  writers  state 
them  somewhat  differently. 

According  to  Prechtl,  if  w=:index  of  refraction,  r=the  radius  of  curvature  of  the 
anterior  surface  of  the  lens,  and  R=radius  of  posterior  surface,  then  for  parallel 
rays  the  form  of  least  aberration  will  be  expressed  by  the  equation 

r 4-}-^ — 

Then  if  the  index  of  refraction  equals  1^,  the  form  of  least  aberration  will  be  ob- 
tained if  the  two  surfaces  have  their  radii  as  1 to  6,  the  side  of  deeper  curvature 
being  towards  the  parallel  rays.  If  the  spherical  aberration  of  such  a lens  in  its 
best  position  be  reckoned  as  unity,  the  aberrations  of  other  lenses  will  be  as 
follows : 

Plano  convex  with  plane  surface  towards  distant  objects,  4.2. 

“ “ “ convex  surface  towards  distant  objects,  1.081. 

Plano  concave  the  same  as  piano  convex. 

Double  convex  or  double  concave  with  both  faces  of  the  same  curvature,  the 
aberration  will  be  1.567. 


CATALOGUE  OF  ACHROMATIC  MICROSCOPES. 


4 


THEORY  OF  THE  MICROSCOPE. 


turned  towards  parallel  rays,  has  its  spherical  aberration  about 

times  its  thickness,  while  in  the  reverse  position  the  aberra- 
tion is  only  about  IjJo  times  the  thickness  of  the  lens. 

7.  Proportionate  Cnrvature.  It  is  thus  seen  that  the 
spherical  aberration  of  a lens  may  be  considerably  reduced,  by 
giving  a proper  proportion  to  the  respective  curvatures  of  its 
two  surfaces,  and  by  turning  the  more  convex  surface  towards 
parallel  rays,  or  the  rays  that  are  nearest  parallel. 

The  thickness  of  a lens  being  very  nearly  as  the  square  of 
its  diameter,  a lens  of  small  diameter  will  have  only  about  one 
fourth  as  much  spherical  aberration  as  a lens  of  the  same  curva- 
ture with  double  its  diameter. 

Hence,  if  only  the  central  portion  of  the  lens  is  used,  the 
aberration  will  be  still  further  diminished.  This  plan  is  adopted 
in  all  the  common  and  cheap  microscopes,  but  the  amount  of 
light  transmitted  is  very  small,  and  other  imperfections  result, 
which  will  be  understood  when  we  treat  of  angular  ajperture, 
(See  15, 16, 17.) 

If  the  refraction  is  performed  by  two  lenses  of  shallow  cur- 
vature, the  aberration  is  less  than  if  the  same  amount  of  refrac- 
tion takes  place  in  a single  lens  of  deep  curvature. 

8.  iVegative  Aberration.  A concave  lens  has  the  same 
amount  of  spherical  aberration  as  a convex  lens,  but  it  takes 
place  in  an  opposite  direction,  and  is  therefore  called  negative 
aberration. 

9.  Aberration  of  Sphericity  : Curvature  of  the  linage. 

When  a flat  object  is  viewed  through  a single  lens,  so  placed 
that  the  central  portion  is  clearly  seen,  the  borders  of  the 
object  appear  indistinct,  and  the  lens  must  be  brought  still 
nearer  in  order  to  view  the  lateral  portions  clearly.  This  effect 
takes  place  chiefly  because  the  lateral  portions  of  the  object 
are  more  distant  from  the  optical  centre  of  the  lens  than  the 
central  portions,  and  partly  because  the  refractive  power  of  the 
lens  is  exerted  more  strongly  on  pencils  of  light,  passing 
obliquely  through  it,  than  on  those  passing  so  that  their  axes 
coincide  with  the  axis  of  the  lens.  For  the  same  reason  an 
image  formed  by  a lens  appears  curved  towards  the  lens 


J.  & W.  GRUNOW  & CO’S  ILLUSTRATED 


CHROMATIC  ABERRATION. 


5 


by  which  it  is  formed,  and  when  this  curved  image  is  viewed 
by  another  lens,  as  in  the  common  compound  microscope,  the 
distortion  of  the  image  is  still  further  increased. 

This  effect,  called  aberration  of  sphericity,  or  curvature  of 
the  image,  is  not  diminished  by  contracting  the  aperture  of  the 
lens,  nor  even  by  dividing  the  refraction  between  two  convex 
lenses,  so  placed  as  to  act  together  as  a single  lens  ; but  when 
the  longitudinal  spherical  aberration  is  corrected,  the  aberra- 
tion of  sphericity,  by  proper  arrangements,  may  be  corrected 
also,  so  that  every  part  of  a flat  object  shall  be  distinctly 
defined  upon  a flat  field.  This  quality,  called  flatness  of  fleld 
is  very  essential  to  a good  microscope. 

10.  Cliroinatic  Aberration  is  another  error  which  arises  in 
the  use  of  a single  lens.  Whatever  be  the  form  of  the  lens, 
the  material  of  which  it  is  composed  does  not  act  uniformly 
upon  the  differently  colored  rays  of  which  white  light  is  com- 
posed, but  separates  each  ray  of  white  light,  falling  obliquely 
upon  its  surface,  into  the  colors  of  the  prismatic  spectrum. 


Fig.  3. 


Let  L L,  Z Z,  Fig.  3,  represent  rays  of  white  light  falling  upon 
a piano  convex  lens.  The  ray  Z Z,  being  nearer  the  border  of  the 
lens  is  strongly  refracted,  and  the  blue  ray  Z B diverges  widely 
from  the  red  ray  Z R,  and  generally,  the  blue  rays  are  brought 
to  a focus  nearer  to  the  lens  than  the  red  rays. 

The  rays  L L,  falling  very  near  the  axis  of  the  lens,  are 
almost  perpendicular  to  the  refracting  surface,  and  hence  the 
colored  rays  of  which  the  white  light  L L is  composed,  are  but 
slightly  separated  from  each  other.  From  this  we  see  that  the 


CATALOGUE  OF  ACHROMATIC  MICROSCOPES. 


THEORY  OF  THE  MICROSCOPE. 


chromatic  dispersion  is  small  near  the  centre  of  the  lens,  and 
like  spherical  aberration,  increases  very  rapidly  towards  the 
borders  of  the  lens. 

In  this  figure,  as  in  Fig.  2,  F f represents  the  longitudinal 
spherical  aberration,  while  the  chromatic  aberration  extends 
over  the  entire  space  from  F to  F.  The  chromatic  aberration 
is  therefore  in  the  same  direction  as  the  spherical  aberration. 

Here  the  narrowest  part  of  the  pencil,  and  hence  the  most 
available  focus,  is  not  at  a 5,  but  at  s s.  In  this  case,  as  with 
spherical  aberration,  if  the  lens  were  placed  with  its  convex 
surface  toward  the  parallel  rays,  it  is  obvious  that  the  longitu- 
dinal chromatic  aberration  F f would  be  diminished,  but  the 
actual  dispersion  of  the  colors  in  each  ray  of  light  would 
remain  unchanged,  except  as  the  change  of  position  causes  a 
slight  alteration  of  the  refractive  power  of  the  lens. 

11.  The  first  efforts  to  diiiiiiiisli  the  Chromatic  Aber- 
ration of  a single  lens  consisted  in  reducing  its  diameter,  as  is 
seen  in  the  cheap  microscopes  so  commonly  found  in  the 
market : in  these  the  orifice  through  which  the  light  passes  is 
exceedingly  small,  and  consequently  the  object  appears  but 
feebly  illuminated.  In  the  compound  microscope  of  such  con- 
struction only  a low  magnifying  power  can  be  used. 

12.  The  second  method  of  diminishing  the  Chromatic 
Aberration  consists  in  employing  two  or  three  lenses  of  shal- 
low curvature  placed  close  together,  by  which  means  the  chro- 
matic and  spherical  aberrations  are  made  as  small  as  is  possible 
with  lenses  composed  of  a single  kind  of  glass.  This  form  of 
lenses,  called  doublets  and  triplets,  is  also  seen  in  cheap  com- 
pound microscopes  of  French  and  German  manufacture.  But 
in  this  form  a considerable  amount  of  chromatic  aberration  still 
remains,  even  when  the  diameter  of  the  lenses  is  quite  small. 

13.  Achromatism.  From  the  preceding  considerations  it 
is  evident,  that  it  is  of  primary  importance,  in  the  construction 
of  a really  efficient  microscope,  that  the  chromatic  and  spheri- 
cal aberration  should  be  entirely  corrected^  but  that  no  such 
correction  can  be  effected  in  a single  lens. 

Hear  the  middle  of  the  last  century,  John  Dollond  of  Lon- 


J.  & W.  GRUNOW  k GO’S  ILLUSTRATED 


ACHBOMATiSM. 


7 


don,  by  combining  a double  convex  lens  of  crown  glass  with  a 
concave  lens  of  flint  glass,  succeeded  in  constructing  an  object- 
glass  for  the  telescope  in  which  the  greater  dispersive  power  of 
the  flint  glass  served  to  correct  the  chromatic  aberration  of  the 
convex  lens  of  crown  glass,  while  a considerable  portion  of  the 
refractive  power  of  the  crown  glass  remained  as  the  efficient 
power  of  the  compound  lens.  This  principle  was  soon  thor- 
oughly established  as  in  every  way  successful  for  forming  achro- 
matic lenses  for  the  telescope.  Efforts  were  made  in  the  early 
part  of  the  present  century  to  improve  the  microscope  in  the 
same  manner.  In  applying  this  principle  to  the  microscope 
several  difficulties  were  encountered.  First,  it  was  found 
extremely  difficult  to  work  the  curves  of  such  small  lenses  with 
sufficient  accuracy  to  insure  freedom  from  chromatic  aberra- 
tion. Secondly,  while  lenses  for  the  telescope  of  large  diame- 
ter constituted  but  very  small  segments  of  spheres,  the  lenses 
requiring  long  foci,  and  being  adapted  to  receive  rays  very 
nearly  parallel,  lenses  for  the  microscope,  receiving  light  radia- 
ting from  a point  very  near  the  lens,  when  made  of  compara- 
tively moderate  diameter  constituted  very  much  larger  seg- 
ments of  spheres  than  the  lenses  of  telescopes.  Thirdly,  it  was 
found  that  when  the  aberrations  of  a convex  lens  of  crown 
glass  were  corrected  by  a concave  lens  of  flint  glass,  if  the 
diameter  of  the  lens  was  enlarged  beyond  very  moderate  limits, 
the  correction  for  the  borders  of  the  lens  became  too  great  in 
proportion  to  the  central  portion,  so  that  there  seemed  to  be  a 
limit,  and  that  a very  small  one,  to  the  available  aperture  of 
achromatic  lenses  constructed  on  this  principle.  So  great,  and 
apparently  invincible,  were  these  difficulties,  that  as  late  as 
1824:,  such  philosophers  as  Biot  and  Wollaston  predicted  that 
the  compound  achromatic  microscope  could  never  be  brought 
to  the  same  degree  of  perfection  as  the  achromatic  telescope. 

14:.  Combinetl  I^enses.  To  overcome  these  difficulties,  dif- 
ferent opticians  combined  two  or  more  compound  lenses,  each 
made  separately  as  nearly  achromatic  as  possible.  By  this 
means  a higher  magnifying  power  was  obtained,  and  a larger 
angular  pencil  of  light  was  transmitted,  though  considerable 


CATALOGUE  OF  ACHROMATIC  MICROSCOPES. 


THEORY  OF  THE  MICROSCOPE. 


light  was  lost  by  reflection  from  the  numerous  refracting  surfa- 
ces employed.  The  lenses  of  each  achromatic  combination 
were  therefore  next  cemented  together,  greatly  diminishing  the 
loss  of  light  by  reflection.  The  compound  lenses,  used  for  the 
formation  of  doublets  and  triplets,  being  each  separately  made 
as  nearly  achromatic  as  possible,  could  be  used  singly  for  low 
powers,  and  two,  three,  or  even  four  of  nearly  the  same  focus 
could  be  screwed  together  in  a tube,  constituting  a compound 
objective  of  high  magnifying  power. 

French  and  German  achromatic  objectives  are  still  made  on 
the  principle  of  allowing  each  achromatic  lens  to  be  used  sepa- 
rately, though  the  several  combinations  of  which  an  objective 
is  composed  have  difierent  magnifying  powers  when  used  alone. 

15.  Angular  Aperture.  It  is  necessary  to  explain  dis- 
tinctly what  is  meant  by  angidar  a^erture^  referring  not  simply 
to  the  diameter  of  the  lens,  but  to  the  angular  divergence  of 
the  extreme  rays  of  the  pencil  of  light,  which  a lens  is  adapted 
to  receive.  It  depends  on  having  the  diameter  of  the  lens 
large  in  proportion  to  the  distance  between  the  lens  and  the 
object,  so  that  a lens  of  short  focus  may  have  a very  large 
angular  aperture^  though  its  absolute  diameter  is  small. 

The  amount  of  light  by  which  any  point  of  an  object  appears 
illuminated,  depends  on  the  angular  dimensions  of  the  trans- 
mitted pencil,  or,  which  is  the  same  thing,  the  angular  aperture 
of  the  object  glass. 

If  the  number  of  rays  of  light  from  any  object  be  insufficient, 
it  cannot  be  seen  even  though  we  employ  a microscope  for  the 
purpose.  With  high  magnifying  powers  the  object  appearing 
greatly  enlarged,  the  light  is  spread  out  over  a large  surface, 
and,  unless  the  amount  of  light  is  proportioned  to  the  magnifying 
power,  the  object  appears  dark  and  imperfectly  illuminated. 

With  a large  angular  aperture  many  delicate  markings 
appear,  which  are  quite  invisible  with  a smaller  aperture.  A 
greater  contrast  is  seen  between  different  parts  of  the  object  with 
a large  angular  aperture,  occasioned  probably  by  the  fact  that 
the  lens  takes  in  a larger  angular  pencil  of  light  from  elevations , 
than  can  issue  from  minute  depressions;  differences  in  the  densi- 
ty of  diflferent  parts  of  an  object  produce  a similar  eftect. 


J.  & W.  GRUNOW  & GO’S  ILLUSTRATED 


LISTER  S DISCOVERIES. 


9 


16.  Oblique  Illumination.  When  an  object  cannot  be 
illuminated  by  a large  angular  pencil  of  light,  nearly  the  same 
effect  may  be  produced  by  employing  very  obliqne  illumina- 
tion, provided  the  object-glass  has  a large  angular  aperture,  so 
as  to  enable  it  to  take  in  a very  oblique  pencil  falling  in  a 
direction  to  be  taken  up  by  one  side  of  the  lens  only.  This  may 
be  illustrated  by  the  well  known  fact,  that  the  numerous  crags 
and  peaks  of  distant  mountains  are  better  distinguished  by  the 
oblique  rays  of  the  setting  or  rising  sun,  than  by  the  more 
direct  meridian  rays  ; and  for  the  same  reason  astronomers  can 
see  and  measure  the  mountains  in  the  moon  better  when  it 
is  new^  or  at  the  quarter^  than  when  it  is  full. 

17.  leister’s  Discoveries.  Among  those  who  have  contrib- 
uted to  the  improvement  of  the  compound  achromatic  micro- 
scope, first  and  foremost  stands  the  name  of  Joseph  Jackson 
Lister,  Esq.  This  observer,  in  1830,  presented  to  the  Eoyal 
Society  a paper,  in  which  he  pointed  out  certain  newly  discov- 
ered properties  of  achromatic  lenses,  by  taking  advantage  of 
which  object-glasses  could  be  constructed,  consisting  of  three 
compound  lenses,  each  having  its  aberrations  more  or  less  cor- 
rected, by  whose  combined  action  a very  large  angular  pencil 
could  be  transmitted,  and  admitting  at  the  same  time,  with  ease 
and  certainty,  of  entire  correction  over  the  whole  field,  perfect 
definition  extending  over  an  entirely  flat  field. 

So  valuable  were  the  discoveries  of  Mr.  Lister,  that  his 
paper  has  formed  the  basis  of  all  the  improvements  in  the 
compound  achromatic  microscope  which  have  been  made  up 
to  the  present  time.  Stimulated  by  Mr.  Lister’s  discoveries, 
practical  opticians  have  carried  the  improvement  of  the  micro- 
scope so  far  that  theory  itself  seems  to  point  to  but  little  further 
improvement  to  be  desired,  and  the  compound  achromatic 
microscope  has  reached  a degree  of  perfection,  hardly  equaled 
by  even  the  most  recent  improvements  of  the  achromatic 
telescope. 

18.  Tlie  essential  requisites  of  a good  Objeet-Olass,  for 

the  compound  microscope,  as  stated  by  Mr.  Lister,  are : First, 
the  transmission  of  a large  focal  pencil,  free  from  all  aberration. 
Second  : that  the  field  of  view  should  be  flat  and  well  defined 


CATALOGUE  OF  ACHROMATIC  MICROSCOPES. 


10 


THEORY  OF  THE  MICROSCOPE. 


throughout.  Third : that  the  light  admitted  should,  as  much  as 
possible,  be  only  such  as  goes  to  form  the  image,  and  it  should 
not  be  intercepted  or  diffused  over  the  field  by  too  many 
reflections. 

19.  Mr.  liister’s  two  preliminary  propositions  are : 

First : that  if  a piano  concave  lens  of  flint  glass  is  employed  to 
correct  the  aberrations  of  a double  convex  lens  of  crown  glass, 
the  correct  centering  of  the  lenses  is  more  easily  effected,  and 
accurate  workmanship  for  a short  focus  is  much  simplified  ; 
though  other  forms  may  be  employed  for  special  purposes.  Sec- 
ond : that  the  concave  and  convex  lenses  should  be  cemented  to- 
gether with  some  substance  permanently  homogeneous,  and  the 
two  curves  should  be  identical  in  form,  and  pressed  close 
together,  so  as  to  leave  between  them  but  a very  thin  layer  of 
cement.  By  cementing  the  lenses  in  this  manner,  the  loss  of 
light  by  reflection  is  diminished  by  nearly  one  half.  Third, 
that  the  compound  piano  convex  lens  thus  formed,  with  curves 
which  render  it  nearly  free  from  aberration,  should  always  be 
used  with  its  flat  surface  towards  the  object  to  be  examined. 

20.  Tlie  principles  discovered  by  Mr.  leister  are  briefly 
stated  as  follows : 

In  lenses  formed  as  above  mentioned,  with  a piano  concave 
lens  of  flint-glass  cemented  to  a double  convex  lens  of  crown, 
rendered  achromatic  by  proper  adjustment  of  the  curves  of  the 
two  lenses,  there  is  some  point  not  far  from  the  principal  focus, 
on  the  plane  side  of  the  compound  lens,  and  situated  in  its 
axis^  from  which  light  falling  upon  the  lens  is  transmitted  free 
also  from  spherical  aberration,  and  emerging  either  nearly 
parallel,  or  tending  to  a conjugate  focus  within  the  tube  of  a 
microscope.  If  the  radiant  point  is  brought  nearer  to  the  lens, 
the  spherical  aberration  will  be  over-corrected ; but  if  the  radi- 
ant continues  to  approach  the  lens,  another  point  will  be  found 
for  which  the  spherical  aberration  is  again  exactly  balanced. 
For  every  radiant  point  still  nearer  to  the  lens,  or  more  distant 
than  the  flrst  point,  the  spherical  aberration  will  be  under- 
corrected. The  two  radiant  points  for  which  the  lens  is  per- 
fectly corrected,  both  for  chromatic  and  spherical  aberration, 


J.  k W.  GRUNOW  & GO’S  ILLUSTRATED 


APLANATIC  OBJECT-GLASSES. 


11 


are  called  the  aplanatic foci.  When  the  longer  aplanatic  focus 
is  used,  the  marginal  rajs  of  an  oblique  pencil  (from  a point 
on  one  side  of  the  axis)  are  distorted  so  that  the  objects  seen  in 
the  borders  of  the  field  appear  fringed  with  a coma  extend- 
ing outwards,  while  the  contrary  effect  of  a coma  directed 
towards  the  centre  of  the  field  is  produced  by  the  rays  from  the 
shorter  focus.  The  correction  of  chromatic  aberration,  like 
that  of  the  spherical,  tends  to  excess  in  the  marginal  rays. 

21.  Aplanatic  Object-Glasses.  These  principles  afibrd  the 
means  of  destroying,  with  the  utmost  certainty,  both  aberrations 
in  a large  focal  pencil,  by  combining  two  or  more  achromatic 
lenses  in  a single  object-glass.  The  rays  of  light  from  an  object 
are  received  by  the  anterior  combination  from  its  shorter 
aplanatic  focus,  and  are  transmitted  to  a second  achromatic 
lens,  of  such  form,  and  so  placed,  as  to  receive  the  rays  in  the 
direction  of  its  longer  aplanatic  focus.  If  the  lenses  are  fixed 
at  this  distance,  the  radiant  point  may  be  moved  backward 
and  forward,  as  required  to  increase  or  diminish  the  length  of 
the  microscope,  without  disturbing  the  balance  of  the  correc- 
tions ; since  the  motion  of  the  radiant  point  produces  opposite 
and  equivalent  errors  in  the  two  compound  lenses.  Slight 
errors  in  color  may  be  destroyed  in  the  same  manner  by  their 
opposites,  and  thus  we  not  only  acquire  fine  correction  for  the 
central  ray,  but  all  coma  of  oblique  pencils  is  destroyed,  and 
the  whole  field  is  rendered  beautifully  flat  and  distinct. 

In  the  application  of  Mr.  Lister’s  principles,  in  order  to 
enlarge  the  angular  aperture  as  much  as  possible,  it  is  found 
better  (as  Mr.  Lister  himself  suggested)  to  retain  in  the  ante- 
rior combination  a certain  amount  of  positive  aberration,  to 
be  corrected  in  the  posterior  combinations,  the  proportionate 
dimensions  of  which  are  somewhat  varied  to  secure  the  best 
effect  in  the  entire  compound  achromatic  objective.  Some- 
times, also,  in  objectives  of  high  power,  the  anterior  and  poste- 
rior combinations  are  each  made  to  consist  of  three  lenses, 
while  the  middle  combination  has  but  two ; but  each  combi- 
nation is  specially  calculated  for  the  place  it  is  to  occupy,  and 
more  or  less  corrected  by  itself,  as  is  found  best  to  secure  the 


CATALOGUE  OF  ACHROMATIC  MICROSCOPES. 


12 


THEOEY  OF  THE  MICKOSCOPE. 


most  delicate  performance  of  the  entire  combination.  Such  a 
combination  of  lenses  is  called  an  aplanatiG  object-glass  or 
objective. 

22.  Cause  of  Superiority  of  EInglisli  and  American  Ob- 
jectives. The  best  English  and  American  opticians  seek  the 
highest  perfection  in  each  objective,  and  hence  their  glasses 
cannot  be  separated  and  arranged  in  new  combinations.  While 
opticians  on  the  continent  of  Europe,  almost  without  exception, 
still  make  their  achromatic  object-glasses  on  the  old  plan  of 
separating  the  combinations,  using  the  several  glasses  sepa- 
rately, or  uniting  them  in  new  combinations. 

Although  French  and  German  achromatic  object-glasses  may 
thus  be  furnished  at  a low  price,  their  performance  cannot 
equal  that  of  the  best  English  and  American  glasses,  which 
will  always  be  preferred  by  those  who  understand  the  philos- 
ophy of  the  microscope,  and  are  capable  of  judging  of  its 
qualities. 

23.  Compound  Achromatic  Microscope.  In  using  the 
achromatic  object-glass  for  microscopial  purposes,  it  is  usuady 
combined  with  another  instrument  known  as  the  eye-^iece^  the 
two,  together,  constituting  the  compound — achromatic — micro- 
scope. The  eye-pieces  used  for  the  microscope  are  the  same  as 
are  employed  for  the  astronomical  telescope,  and  consist  of 
two  kinds. 

24.  The  Positive  Eye-piece  consists  of  two  piano  convex 
lenses,  with  their  convex  sides  turned  towards  each  other,  and 
set  at  such  a distance  that  their  compound  focus  is  in  front  of 
the  first  lens.  An  eye-piece  of  this  construction  has  less  achro- 
matic and  spherical  aberration  than  any  single  lens  of  the  same 
power,  but  as  it  causes  some  renewed  coloring  and  distortion  of 
the  image  formed  by  the  object-glass,  it  is  only  used  in  the 
microscope  for  special  purposes. 

25.  The  Negative  Eyc-picce,  invented  by  Huyghens,  for 
the  telescope,  is  so  named  because  it  requires  the  image  to  be 
formed  behind  the  first  lens,  the  second,  or  eye-lens,  only  being 
employed  to  view  the  image.  Two  piano  convex  lenses  are 
placed  with  their  plane  surfaces  towards  the  eye,  at  a distance 


J.  & W.  GRUNOW  & GO’S  ILLUSTRATED 


COMPOUMD  ACHROMATIC  MICROSCOPE. 


13 


from  each  other,  equal  to  half  the  sum  of 
their  focal  lengths,  and  with  a stop  or 
diaphragm  placed  midway  between  the 
lenses.  Huyghens  intended  this  eye-piece 
to  diminish  the  spherical  aberration,  like 
the  positive  eye-piece,  and  especially  to 
enlarge  the  field  of  view,  both  valuable 
qualities,  but  he-  was  not  aware  of  the 
most  important  excellence  of  his  inven- 
tion. It  was  reserved  for  Boscovich  to 
show  that  he  had  by  this  important 
arrangement  accidentally  corrected  a 
great  part  of  the  chromatic  aberration, 
as  will  be  shown  hereafter.  (See  30.) 
The  negative  eye-piece  is  therefore  the 
one  generally  employed  for  the  micro- 
scope. 

26.  A Section  of  a modern  Com- 
pound Achromatic  Microscope,  is 

shown  at  Fig.  4,  where  0 is  an  object, 
and  above  it  is  seen  the  triple  achromatic 
objective.  The  lenses  E E,  and  F F,  con- 
stitute the  negative  eye-piece  invented 
by  Huyghens.  The  piano  convex  lens 
E E,  is  called  the  eye-glass,  F F is  the 
field-glass,  and  between  them,  at  B B, 
is  a dark  stop  or  diaphragm. 

The  course  of  light  is  shown  by  the 
three  rays  drawn  from  the  centre,  and 
three  from  each  end  of  the  object  O ; 
these  rays,  if  not  prevented  by  the  lens  F 
F,  or  the  diaphragm  at  B B,  Avould  form 
an  image  at  A A ; but  as  they  meet  with 
the  lens  F F,  in  their  passage,  they  are 
converged  by  it,  and  meet  at  B B,  where 
the  diaphragm  is  placed  to  intercept  all 


Fig.  4. 


CATALOGUE  OF  ACHROMATIC  MICROSCOPES. 


14 


THEOKY  OF  THE  MICROSCOPE. 


the  light,  except  that  required  for  the  formation  of  a perfect 
image,  and  to  limit  the  field  of  view  to  such  an  aperture  as 
will  be  well  defined  when  viewed  with  the  eye-lens  E E. 

The  image  formed  at  B B,  is  further  magnified  by  the  eye- 
lens,  as  if  it  were  an  original  object.  The  triple  achromatic 
object-glass,  constructed  on  the  principles  discovered  by  Mr. 
Lister,  though  capable  of  transmitting  large  angular  pencils, 
and  corrected  as  to  its  own  errors  of  spherical  and  chromatic 
aberration,  would,  nevertheless,  be  incomplete  without  some 
special  adaptation  to  prevent  renewed  aberrations  distorting  the 
image  as  transmitted  and  viewed  by  the  eye-piece. 

The  property  of  the  negative  eye-piece,  pointed  out  by  Bos- 
covich,  most  admirably  meets  this  condition  ; for  although  it 
would  not  be  free  from  aberrations  when  used  alone,  yet,  when 
used  as  an  eye-piece,  in  connection  with  an  objective,  its  effect 
upon  converging  pencils  is  such  that  the  aberrations  produced 
by  the  field-lens  are  very  nearly  balanced  by  opposite  aberra- 
tions in  the  eye-lens,  resulting  from  the  fact  that  it  is  situated 
on  the  opposite  side  of  the  image  formed  between  the  two 
lenses,  and  because  the  light  from  the  object  falls  only  upon 
those  parts  of  the  field-lens  F F,  which  are  best  adapted  to 
transmit  it  free  from  error. 

27.  Enlarged  Section  of  tlie  Compound  Microscope.  A 

more  complete  view  of  the  action  of  the  several  parts  of  the 
compound  achromatic  microscope,  is  given  in  Fig.  5,  the  lenses 
being  represented  on  an  enlarged  scale.  A A,  M M,  P P,  repre- 
sent the  three  compound  lenses  of  which  the  achromatic  ob- 
jective is  composed.  F F is  the  field-lens,  and  E E the  eye-lens 
of  the  negative  eye-piece. 

Three  raj^s  drawn  from  the  centre  of  the  object  O,  and  three 
from  each  extremity,  show  the  course  of  both  direct  and 
oblique  pencils.  It  is  impossible  in  the  space  allowed  to  the 
figure,  to  show  the  separate  action  of  each  concave  and  convex 
lens,  but  only  the  action  of  the  objective  considered  as  a whole. 

First : the  axial  rays,  both  of  direct  and  oblique  pencils, 
cross  at  some  point  which  constitutes  the  optical  center  of  the 
compound  objective,  and  emerge  from  the  posterior  lens  in  the 
same  direction  they  pursued  on  leaving  the  object ; the  lateral 


J.  & W.  GRUNOW  & GO’S  CATALOGUE. 


Fig.  5. 


displacement  which 
they  undergo  is  too 
small  to  be  shown  in 
the  figure,  and  is 
altogether  unimport- 
ant. 

Second  : the  ex- 
treme rays  from  each 
pencil  cross  each  oth- 
er in  the  borders  of 
the  objective. 

Third : the  rays  is- 
sue from  the  pos- 
terior combination 
slightly  over-correct- 
ed for  color,  so  that 
if  no  other  lens  in- 
tervened a blue  im- 
age would  be  formed 
at  B B,  and  a red 
image  at  K K,  wliile 
other  images,  with 
intermediate  colors, 
would  fill  up  all  the 
space  between  B B 
and  K E. 

In  a single  convex 
lens,  as  we  have  seen 
in  § 10,  the  blue  im- 
age is  formed  nearer 
to  the  lens  than  the 
red. 

This  slight  over- 
correction of  the  ob- 
ject-glass,really  much 
less  than  shown  in  the 

Note, — The  course  of 
rays  shown  is  not  strictly 
accurate,  but  such  as  to 
show  correctly  the  princi- 
ples referred  to  in  the  de- 
scription. 


16 


THEORY  OF  THE  MICROSCOPE. 


figure,  is  purposely  produced,  to  balance  the  small  amount 
of  aberration  which  remains  otherwise  uncorrected  in  the  nega- 
tive eye-piece.  The  aberration  of  sphericity  connected  with 
the  object-glass,  is  slightly  under-corrected,  as  is  shown  by 
the  images  B B and  R B,  which  are  turned  with  their  concave 
sides  towards  the  object-glass. 

28.  Action  of  the  Negative  Eye-Piece.  The  field  lens  F 
F,  bends  the  lateral  pencils  inwards,  forming  images  nearer 
and  smaller  than  would  have  been  formed  without  its  action. 
This  action  of  the  field-lens  diminishes  somewhat  the  magnify- 
ing power  of  the  instrument,  but  it  enlarges  the  field  of  view, 
and  it  is  hence  called  the  field-lens. 

Secondly  : the  field-lens  refracting  the  blue-rays  more  strongly 
than  the  red,  the  blue  and  red  images  are  brought  nearer 
together,  as  shown  at  B'  B',  R'  R'. 

Thirdly  : the  blue  rays  being  bent  out  of  their  course  by  the 
field-lens  more  than  the  red,  the  blue  image  becomes  so  much 
smaller  than  the  red  that  both  images  are  seen  in  the  same 
direction,  as  shown  by  the  lines  of  sight,  a (?,  and  a'  O",  Fig.  5, 
which,  by  their  intersection  at  the  optical  centre  of  the  eye- 
lens,  form  the  visual  angle  under  which  the  magnified  image  of 
the  object  is  seen,  as  though  situated  at  O O'  O". 

Fourthly : the  field-lens  being  thicker  in  its  centre  than  at  the 
edges,  and  the  rays  from  the  centre  of  the  object  all  falling 
upon  the  central  portion  of  the  field-lens,  while  the  pencils 
from  the  extremities  of  the  object  fall  upon  the  borders  of  the 
field-lens,  the  curvature  of  the  image  is  reversed,  and  the  images 
B'  B',  R R',  have  their  concave  surfaces  turned  towards  the 
eye-lens,  and  are  in  the  exact  condition  to  appear  as  a single 
straight  image  at  O O'  O",  when  viewed  through  the  eye- 
lens  E E. 

Fifthly:  the  lines  a O,  and  a'  0'\  passing  through  the  optical 
centre  of  the  eye-lens,  and  through  the  extremities  of  the  real 
images  B'  B',  and  R'  R',  if  extended  back  to  the  limit  of  dis- 
tinct vision,  (which  is  generally  reckoned  at  ten  inches,)  show 
the  direction  and  apparent  size  of  the  magnified  image,  as  it 
appears  to  the  eye. 


J.  & W.  GRUNOW  & GO’S  ILLUSTRATED 


NEGATIVE  EYE-PIECE  NEARLY  ACHROMATIC. 


17 


Sixthly : the  red  and  blue  rays,  after  being  refracted  by  the 
eye-lens,  arrange  themselves  in  such  relation  to  the  lines  a 0 
and  a'  0'\  that  they  all  appear  to  proceed  from  a single  straight 
image,  entirely  free  from  both  chromatic  and  spherical  aber- 
ration. The  rays  B 0 and  R 0 not  actually  coincide  in 
position,  but  they  appear  to  emanate  from  the  same  point, 
while  other  rays,  emanating  from  the  same  point  in  the  object, 
so  over-lie  these  as  to  unite  in  every  position  all  the  colors  of 
the  spectrum,  giving  perfectly  white  light  and  achromatic  vis- 
ion of  the  object. 

29.  Advantage  of  over-correcting  tlie  Object-Olass. 

The  eye-lens  E E has  its  focus  for  red  rays  longer  than  its 
focus  for  blue  rays.  If  the  object-glass  had  not  been  over- 
corrected, the  action  of  the  field-lens  would  have  caused  the 
blue  image  to  be  formed  at  5,  and  the  red  image  at  but  the 
over-correction  of  the  object-glass  has  placed  the  blue  image 
as  much  nearer  to  the  eye-lens  than  the  red,  as  is  required  by 
the  diiference  between  its  foci  for  blue  and  red  rays,  and  the 
curvature  of  the  images  which  has  been  reversed  by  the  field- 
lens,  just  equals  the  aberration  of  sphericity  of  the  eye-lens,  so 
that  it  appears  to  the  observer  free,  also,  from  this  species  of 
error,  as  shown  by  the  magnified  straight  image  which  the  eye 
sees  situated  at  0 O". 

30.  IVegative  Eye-piece  nearly  Achromatic.  Let  us 
now  examine  the  action  of  the  lenses  F F and  E E,  a little 
more  particularly.  The  pencil  of  rays  from  the  centre  of  the 
object  is  so  condensed  by  the  object-glass  AMP,  that  it  occu- 
pies but  a small  space  about  the  central  portion  of  the  field- 
lens  F F.  The  border  rays  of  this  pencil,  after  passing  the 
field-lens,  cross  each  other,  the  red  rays  in  the  red  image  K'  E', 
and  the  blue  rays  in  the  blue  image  B',  and  after  thus  cross- 
ing, they  impinge  upon  the  eye-lens  each  ray  on  the  opposite 
side  of  the  axis  from  what  it  was  when  refracted  by  the  field- 
lens  ; thus  the  lateral  chromatic  aberration  of  the  field-lens  will 
be  nearly  corrected  by  the  eye-lens,  but  as  the  eye-lens  has  a 
shorter  focus  than  the  field-lens,  its  chromatic  aberration  will 
be  in  excess. 


CATALOGUE  OF  ACHROMATIC  MICROSCOPES. 
2 


18 


THEORY  OF  THE  MICROSCOPE. 


In  the  same  manner  it  might  be  shown  that  the  spherical 
aberration  of  the  lateral  pencils  will  be  very  small,  because 
they  have  been  so  condensed  by  the  action  of  the  object-glass 
that  a pencil  of  light  from  any  point  in  the  object,  occupies 
but  a very  small  space  in  either  the  field-lens  or  eye-lens. 

Examining  the  pencil  from  one  extremity  of  the  object, 
which,  without  the  intervention  of  the  field-lens,  would  have 
converged  to  R and  B,  we  see  that  its  central  ray,  represented 
by  the  smooth  white  line  which  passed  through  the  optical  cen- 
tre of  the  object-glass,  arrives  at  the  field-lens  uncolored,  but 
is  there  divided  into  blue  and  red  rays,  (and  other  colors  not 
represented  in  the  figure.)  The  blue  ray,  which  is  refracted 
more  strongly  than  the  red,  falls  nearer  the  centre  of  the  eye- 
lenSy  where  its  refractive  power  is  small,  while  the  red  ray, 
which  is  feebly  refracted,  falls  nearer  the  border  of  the  eye- 
lens,  where  the  curvature  of  the  lens  increases  its  refraction,  so 
that  it  emerges  from  the  eye-lens  very  nearly  parallel  with  the 
blue  ray  from  which  it  was  separated  by  the  field-lens.  In  the 
same  manner  all  the  red  rays  occupy  the  parts  of  the  eye-lens 
where  the  refractive  power  of  the  lens  is  greater  than  at  the 
points  occupied  by  the  corresponding  blue  rays.  Thus  the 
chromatic  aberration  of  the  field-lens  is  very  nearly  corrected 
by  the  eye-lens.  This  is  the  property  of  the  negative  eye-piece 
pointed  out  by  Boscovich.  The  excess  of  chromatic  aberration 
in  the  eye-lens  is  balanced  by  a small  amount  of  over-correction 
in  the  object-glass. 

31.  Use  of  tlie  term  Acliroiiiatic  Objective.  From  what 
has  been  stated,  it  is  obvious  that  we  mean  by  an  achromatic 
object-glass,  or  objective,  one  in  which  the  usual  order  of  disper- 
sion is  so  far  reversed,  that  the  light,  after  undergoing  the  singu- 
larl}^  beautiful  series  of  changes  effected  by  the  eye-piece,  shall 
come  uncolored  to  the  eye.  IIo  specific  rules  can  be  given  for 
producing  these  results.  Close  study  of  the  formulae  for  achro- 
matism, and  accurate  calculation  of  the  due  proportions  of  all 
the  parts  of  the  instrument,  are  essential,  but  the  principles 
must  be  brought  to  the  test  of  repeated  experiment.  Nor  will 
the  experiments  be  of  any  value,  unless  the  curves  be  most  ac- 


J.  & W.  GRUNOW  & GO’S  ILLUSTRATED 


ABERRATION  PRODUCED  BY  GLASS  COVER. 


19 


curatelj  measured  and  worked,  and  the  lenses  centered  and  ad- 
j listed  with  a degree  of  precision  quite  inappreciable  to  those 
who  are  not  practically  acquainted  with  this  kind  of  work. 

32.  Aberration  Produced  by  Glass  Cover.  When 
achromatic  object-glasses  of  considerable  angular  aperture  are 
accurately  tested,  it  is  found  that  so  perfect  are  the  corrections, 
so  completely  are  the  errors  of  sphericity  and  dispersion  bal- 
anced or  destroyed,  that  if  the  glass  has  been  adjusted  for  view- 
ing an  object  uncovered,  a plate  of  the  thinnest  glass  or  mica 
placed  over  the  object,  sensibly  disturbs  the  correction,  produ- 
cing colored  fringes  and  indistinctness  of  outline  in  all  parts  of 
the  field,  so  that  some  new  method  of  adjustment  is  required. 
This  defect  (if  that  should  be  called  a defect,  which  arose  out 
of  improvement,  or  which  was  only  rendered  sensible  by  the 
great  improvement  given  to  object-glasses,  as  the  result  of  Mr. 
Lister’s  discovery)  was  first  discovered  by  Mr.  Koss,  who  im- 
mediately suggested  a ready  means  of  correcting  it. 

The  aberration  referred  to,  which  is  produced  upon  diverg- 
ing rays  by  a piece  of  fiat  and  parallel  glass,  such  as  would  be 
used  for  covering  an  object,  will  be  understood  by  the  follow- 
ing figure : 

Let  GGGG  be  the  re-  ^ Fig.  6. 

fracting  medium,  or  a 
piece  of  glass  covering 
the  object  0,  and  O P 
the  axis  of  the  pencil 
perpendicular  to  the 
fiat  surfaces,  O T,  a 
ray  near  the  axis,  and 
O T',  the  extreme  ray 
of  the  pencil  incident 
on  the  under  surface  of 
the  glass:  then  T P, 

T'  K'  will  be  the  direc- 
tions of  the  rays  in  the  medium,  and  K E,  P'  E',  those  of  the 
emergent  rays. 

Now  it  the  course  of  these  rays  is  continued  backward,  as 


CATALOGUE  OF  ACHROMATIC  MICROSCOPES. 


20 


THEORY  OF  THE  MICROSCOPE. 


shown  bj  the  dotted  lines,  they  will  be  found  to  intersect  the 
axis  at  different  distances,  X and  Y,  from  the  surface  of  the 
glass ; the  distance  X Y,  is  the  aberration  produced  by  the 
medium,  which,  as  before  stated,  interferes  with  the  previously 
balanced  aberrations  of  the  several  lenses  composing  the  object- 
glass.  The  spherical  aberration  thus  produced  by  the  thin 
glass,  or  other  medium  covering  the  object,  being  in  a direc- 
tion the  opposite  of  that  produced  by  a single  convex  lens, 
is  called  negative  aberration.  Chromatic  aberration  is  pro- 
duced by  the  thin  glass  in  the  same  direction.  The  effect  ob- 
served by  the  eye,  is,  that  lines  are  not  so  sharply  defined,  and 
the  outline  of  any  object  appears  bordered  with  larger  and 
thicker  fringes,  with  colors  of  the  secondary  spectrum  upon  the 
borders  of  the  object. 

33.  Objective  corrected  for  Olass  Cover.  If  an  object- 
glass  constructed  as  shown  at  A M P,  Fig.  5,  have  its  anterior 
combination  A,  somewhat  under-corrected,  so  as  to  leave  a 
degree  of  positive  aberration,  and  the  middle  and  posterior 
combinations  M P,  have  together  an  excess  of  negative  aber- 
ration, so  as  to  balance  the  under-correction  of  the  anterior  com- 
bination, then  the  positive  aberration  of  this  latter  combination 
will  act  more  powerfully  upon  the  other  two  when  brought  as 
near  to  them  as  possible,  and  less  powerfully  when  the  distance 
between  them  is  increased.  When  the  three  combinations  are 
in  close  contact,  their  common  focus,  and  consequently  the 
object,  is  at  the  greatest  distance  from  the  front  lens  ; the  rays 
from  the  object  are  therefore  diverging  from  a j^oint  at  a greater 
distance  than  when  the  lenses  are  separated.  A lens  bends  the 
rays  more  that  diverge  from  a distant  object,  and  of  course  its 
aberration  is  then  greater,  hence  the  anterior  combination  A,  will 
have  a greater  positive  aberration,  and  act  more  strongly  upon 
the  other  combinations  M and  P,  and  this  effect  will  vary  with 
the  distance  between  the  anterior  and  other  combinations. 

When,  therefore,  the  correction  of  the  entire  objective  is  ef- 
fected for  an  uncovered  object,  with  a certain  distance  be- 
tween the  anterior  and  middle  combinations,  if  they  are  then 
brought  into  contact,  the  distance  between  the  anterior  com- 


J.  & W.  GRUNOW  & GO’S  ILLUSTRATED 


NAMES  APPLIED  TO  OBJECT-GLASSES. 


21 


bination  and  the  object  will  be  increased ; hence  the  ante- 
rior combination  will  act  more  powerfully,  and  the  entire  com- 
pound objective  will  have  a certain  excess  of  positive  aberra- 
tion. 

Now  as  a piece  of  flat  and  parallel  glass  placed  over  an  ob- 
ject produces  chromatic  and  spherical  aberration,  both  nega- 
tive, it  is  evident  that  it  may  be  corrected  by  diminishing  the 
distance  between  the  anterior  and  middle  combinations  of  the 
objective,  till  the  positive  aberration  thereby  produced  in  the 
object-glass  balances  the  negative  aberration  caused  by  the 
medium. 

This  correction  is  essential  to  the  performance  of  object-glass- 
es of  large  angular  aperture.  Glasses  of  moderate  angular 
aperture  may  have  their  corrections  balanced  for  a medium 
thickness  of  cover,  and  then  they  will  perform  very  well  if  the 
thickness  of  glass  or  other  medium  covering  the  object  is  va- 
ried within  moderate  limits.  The  mechanical  arrangement  of 
the  object-glass,  by  which  [the  correction  is  made,  to  adapt  it 
for  viewing  objects  covered  with  different  thicknesses  of  glass 
or  fluid,  is  fully  explained  at  section  58. 

34.  Tlie  magnifying  power  of  the  ]TIicro§cope  is  varied 
by  the  use  of  Eye-pieces  of  different  poAvers,  which  are 
designated  as  Nos.  1,  2 and  3,  No.  1 having  the  least  magni- 
fying power.  Nos.  1 and  2 are  those  generally  furnished  with 
our  microscopes.  No.  3 is  of  still  higher  power,  and  is  only 
furnished  when  specially  ordered. 

35.  Magnifying  power  varied  by  Draw-tube.  The 
power  of  the  microscope  may  be  greatly  increased  not  only  by 
using  different  eye-pieces  and  object-glasses,  but  also  by  increas- 
ing the  distance  between  them  by  means  of  the  draw-tube,  with 
which  the  better  instruments  are  supplied. 

36.  Names  applied  to  Object-Glasses.  Object-glasses  are 
usually  distinguished  by  the  focal  length  of  a single  lens  that 
would  give  the  same  linear  magnifying  power ; but  the  dis- 
tance between  the  object  and  the  anterior  combination  of  a 
compound  achromatic  objective,  is  far  less  than  its  designation 
would  imply,  for  its  optical  centre,  from  which  its  true  focus 


CATALOGUE  OF  ACHROMATIC  MICROSCOPES. 


22 


THEORY  OF  THE  MICROSCOPE. 


should  be  reckoned,  is  at  a considerable  distance  behind  the 
exterior  surface  of  the  anterior  lens. 

37.  Foci  of  liigh  powers  iiicoiiveiiiently  near  to  tlie 
Otojective.  In  improved  triple  achromatic  objectives  of  high 
magnifying  power  and  mry  large  angular  aperture,  the  diame- 
ter is  only  sufficient  to  admit  the  proper  pencil ; the  convex 
lenses  are  wrought  to  an  edge,  and  the  concave  lenses  have 
only  sufficient  thickness  to  support  their  figure ; consequently 
the  entire  combination  is  the  thinnest  possible,  and  yet  the 
focus  of  the  compound  achromatic  objective  is  so  near  the 
anterior  surface  of  the  front  lens  that  an  object  can  only  be 
covered  with  the  thinnest  film  of  glass  which  can  be  obtained, 
when  it  is  viewed  with  the  highest  powers. 


J.  k W.  GRUNOW  & GO’S  ILLUSTRATED 


ARRANGEMENT  FOR  INCLINING  THE  INSTRUMENT. 


23 


CHAPTER IL 

MECHANICAL  PORTION  OF  THE  MICROSCOPE. 

38.  Modern  lmprovemeiit§  in  the  optical  portion  of  the 
microscope  have  rendered  necessary  corresponding  improve- 
ments in  the  mechanical  arrangement  and  support  of  the 
several  parts,  in  order  to  secure  facility  of  manipulation  and 
freedom  from  tremor,  or  vibration,  which  might  disturb  the 
steadiness  of  distinct  observation. 

The  microscope  stands  or  mechanical  portion^  consists  of  the 
base  or  foot,  the  stage,  and  compound  body  with  its  support, 
including  the  mechanical  arrangements  for  effecting  the  requi- 
site adjustment  of  all  the  parts. 

39.  Base.  “The  instrument  (says  Dr.  Beale)  should  stand 
firmly,  whether  the  body  be  inclined  or  arranged  in  a vertical 
position,  and  not  the  slightest  lateral  movement  should  be  com- 
municated to  the  body  of  the  microscope,  when  the  focus  is 
altered  by  turning  the  adjustment  screws.  The  base  should 
be  sufficiently  heavy  to  give  steadiness,  and  should  be  made 
either  of  a tripod  form,  or  placed  upon  three  small  feet,  (a 
simple  means  of  insuring  steadiness  not  attended  to  in  many 
of  the  foreign  instruments.)” 

40.  Arrangement  for  Inclining  the  Instrument.  The 

instrument  ought  to  be  provided  with  an  arrangement  by  which 
it  may  be  placed  in  a horizontal  position,  or  inclined  at  any  an- 
gle, for  the  convenience  of  drawing  with  the  camera,  or  meas- 
uring objects  with  the  aid  of  that  instrument,  or  to  give  an 
easy  position  to  the  observer.  When  the  eye  looks  directly 
downwards  into  the  microscope,  the  fluids  collecting  on  the 


CATALOGUE  OF  ACHROMATIC  MICROSCOPES. 


24 


MECHANICAL  PORTION  OF  THE  MICROSCOPE. 


centre  of  the  cornea  impair  the  distinctness  of  vision  ; hence 
difficult  test  objects  should  always  be  viewed  with  the  instru- 
ment inclined.  An  inclination  of  about  55°  to  the  horizon 
generally  affords  the  easiest  position  for  protracted  obser- 
vation. 

‘‘It  is  matter  of  surprise,  (says  Dr.  Carpenter,  in  his  treatise 
on  the  microscope,)  that  opticians  on  the  continent  of  Europe 
have  generally  neglected  this  very  necessary  convenience  in 
the  arrangement  of  the  microscope.” 

41.  Stage.  The  stage  should  be  large  enough  to  support 
conveniently  any  object,  and  to  bring  any  portion  of  the  slide, 
on  which  the  object  is  placed,  into  the  field  of  view.  The 
stages  of  many  European  microscopes  are  inconveniently 
small.  Spring  or  sliding  clips  are  required  to  retain  the 
object  in  place  when  the  instrument  is  inclined.  If  a mov- 
able stage  is  attached  to  the  microscope,  the  movement  should 
be  smooth  and  steady  in  every  direction,  and  the  stage 
should  remain  steady  as  it  is  placed  when  the  instrument  is 
inclined. 

42.  Adjustment  ©f  Focus.  The  mode  of  effecting  the  fo- 
cal adjustment  should  be  such  as  to  allow  a free  range  of  tw^o 
or  three  inches,  to  suit  the  focus  of  any  object-glass,  and  there 
should  be  an  arrangement  for  obtaining  a delicate  adjustment 
in  every  part  of  the  range.  To  secure  these  objects  a coarse 
adjustment,  by  rack  and  pinion  movement,  is  generally  em- 
ployed, and  a delicate  fine  adjustment  by  a screw'  acting  upon 
the  end  of  a lever. 

43.  Simplicity  and  Facility  of  Adjustment  should  be 
secured  in  every  part,  and  the  instrument  should  be  so  arranged 
in  its  case  that  it  can  be  taken  out  and  fitted  for  observation 
with  as  little  labor  as  possible.  Many  interesting  objects  are 
likely  to  pass  unnoticed,  if  much  time  is  required  to  unpack 
and  adjust  the  instrument  for  observation. 

44.  Description  of  Microscope  Stands.  Since  w e have 
been  engaged  in  the  manufacture  of  microscopes,  we  have 
carefully  examined  the  most  approved  forms  of  English  and 


J.  & W.  GRUNOW  & GO’S  ILLUSTRATED 


EDUCATIONAL  MICROSCOPE. 


25 


American  microscopes,  and  having  had  the  advice  of  several 
of  the  most  eminent  American  microscopists,  in  regard  to  im- 
provements in  the  me- 
chanical  arrangement 
of  our  instruments, 
we  have  devised  and 
adopted  a variety  of 
forms  and  sizes  of  mi- 
croscope stands,  adapt- 
ed to  the  use  of  every 
class  of  observers,  and 
suited  to  the  means  of 
purchasers. 

For  steadiness  of  sup- 
port, freedom  from  tre- 
mor, convenience  and 
simplicity  of  arrange- 
ment, and  finished 
workmanship,  we  are 
confident  that  our  mi- 
croscopes will  prove 
satisfactory  to  all  who 
may  use  them. 

45.  Educational 
Microscope.  This  in- 
strument is  mounted  on 
a firm  tripod,  with  up-  no.  i.  educational  microscope. 

rights  of  japanned  cast-  when  arranged  for  use. 

iron.  A solid  limb  of  japanned  cast-iron  supports  the  stage 
and  the  body  of  the  instrument,  and  being  attached  to  the 
uprights  by  a trunnion  joint,  it  allows  the  instrument  to  be  in- 
clined at  any  angle.  The  body  of  the  instrument  slides  easily 
and  steadily  in  a firm,  but  elastic  brass  cylinder,  attached  to 
the  japanned  limb,  by  which  means  it  is  readily  adjusted  to 
any  desired  focus.  The  stage  is  two  by  three  inches,  having 
spring  clips  to  retain  the  object  in  place  when  the  microscope 


CATALOGUE  OF  ACHROMATIC  MICROSCOPES. 


26 


MECHANICAL  PORTION  OF  THE  MICROSCOPE. 


is  inclined.  A fine  screw,  with  a milled  head,  at  the  right  of 
the  stage,  gives  a fine  adjustment  to  the  focus. 

Below  the  stage  is  a diaphragm  plate,  with  orifices  of  differ- 
ent sizes  to  regulate  the  illumination,  and  a space  between  the 
largest  and  smallest  orifices  to  exclude  all  the  light,  and  give  a 
dark  background  for  viewing  opaque  objects. 

A concave  mirror  an  inch  and  a half  in  diameter,  suspended 
by  a cradle  joint,  and  movable  in  every  direction,  is  used  for 
illuminating  the  object.  The  mirror  is  so  attached  to  the  axis 
of  the  instrument,  by  a movable  arm,  that  it  can  be  turned  so 
as  to  give  very  oblique  light. 

This  instrument  is  generally  supplied  with  two  eye-pieces, 
and  with  one’  inch  and  one  quarter  inch  objectives  of  second 
quality,  giving  four  magnifying  powers,  varying  from  40  to 
350  diameters. 

This  microscope  is  designed,  as  its  name  implies,  for  educa- 
tional purposes,  for  schools,  private  families,  and  for  young 
people  generall3^  Farmers,  mechanics  and  merchants,  who 
desire  to  devote  some  of  their  leisure  hours  to  intellectual  im- 
provement, or  to  the  investigation  of  those  branches  of  natural 
science  more  or  less  connected  with  their  several  avocations, 
will  find  this  at  once  a cheap,  substantial  and  efiicient  micro- 
scope. 

As  it  is  very  steady  and  delicate  in  its  adjustments,  and  can 
be  used  with  the  higher  powers,  the  man  of  science  will  often 
find  it  a convenient  substitute  for  the  larger  microscopes,  when 
a more  portable  instrument  is  required  for  special  purposes. 

46.  The  Students  Microscope,  shown  in  figure  8,  is  mounted 
on  a firm  tripod  base,  with  uprights  of  japanned  cast-iron,  and 
moves  freely  on  trunnions,  so  that  it  may  be  inclined  at  any 
angle.  It  has  a plain  stage  three  by  four  inches,  furnished 
with  spring  clips,  which  retain  the  object  on  the  stage  when 
the  instrument  is  inclined,  and  yet  allow  it  to  be  ynoved  freely 
in  any  direction.  Plane  and  concave  mirrors  are  set  in  the 
same  frame,  and  can  be  rotated  in  any  direction,  or  moved  up- 
ward and  downward  on  a sliding  support.  The  adjustment  o 


J.  & W.  GRUNOW  & GO’S  ILLUSTRATED 


ANOTHER  FORM  OF  STTJDENt’s  MICROSCOPE. 


27 


focus  is  performed  by  a tine 
rack  and  pinion  movement,  by 
turning  either  or  both  of  the 
milled  heads  at  the  back  of 
the  stage. 

The  body  of  the  instrument 
is  attached  to  a strong  arm  by 
a bayonet  joint,  and  can  be 
easily  removed  to  allow  of 
packing  into  a small  case. 

When  the  body  is  thus  remov- 
ed, it  can  be  used  as  a dissect- 
ing microscope  by  inserting 
into  the  arm  lenses  adapted  to 
that  purpose. 

This  instrument  is  usually 
supplied  with  two  eye-pieces, 
and  with  1 inch  and  i inch 
objectives  of  second  quality, 
giving  four  magnifying  pow- 
ers, varying  from  40  to  350  di- 
ameters. Other  object-glasses  | 
and  accessory  apparatus  can  ' 
be  furnished,  to  suit  the  pur- 
chaers. 

This  instrument  is  well  adapt- 
ed for  the  use  of  students  in  botany  and  natural  history,  and 
also  for  schools  and  private  families,  who  take  an  interest  in 
the  wonders  revealed  by  the  microscope. 

47.  Aiiotlier  form  of  Stiident^s  Microscope  is  shown  at 
tigure  9,  mounted  on  a tripod  base  like  No.  2,  tigure  8,  with  a 
trunnion  joint  to  incline  it  at  any  angle.  It  has  plane  and 
concave  mirrors,  mounted  in  the  same  manner  as  in  No.  2. 
The  body  of  this  microscope  slides  smoothly  and  easily  in  a 
strong  cylindrical  support.  A screw  with  a milled  head. 


NO.  2.  STUDENT’S  MICROSCOPE. 
Twelve  inches  high  when  arranged  for  use. 


CATALOGUE  OF  ACHROMATIC  MICROSCOPES. 


28 


MECHANICAL  PORTION  OF  THE  MICROSCOPE. 


48. 


seen  behind  the  stage, 
acts  upon  a lever,  which 
effects  a delicate  fine 
adjustment  of  the  focus. 
This  movement  adapts 
this  form  of  the  stu- 
dent’s microscope  to  be 
used  with  the  higher 
powers.  The  stage  is 
three  by  four  inches,  and 
has  spring  clips  for  se- 
curing the  object.  A 
circular  plate  attached 
beneath  the  stage,  and 
carefully  centered,  af- 
fords attachment  to  the 
polarizer  and  achromat- 
ic condenser,  and  any 
other  apparatus  which 
requires  to  be  attached 
beneath  the  stage. 

Every  kind  of  access- 
ory apparatus  may  be 
used  with  this  instru- 
ment. Medical  students, 
and  all  others  who  can- 
not afford  the  larger  in- 
struments, will  find  this 
microscope  well  adapt- 
ed to  almost  every  kind 
of  observations,  even 
with  the  highest  powers. 
Tlie  Student’s  I..argcr  Microscope,  shown  in  figure 


NO.  8.  STUDENT’S  MICROSCOPE. 
Thirteen  inches  high  when  arranged  for  use. 


10,  is  made  of  the  dimensions  agreed  upon  by  microscopists  as 
most  convenient  for  general  use,  and  fitted  for  the  application 
of  accessory  apparatus  of  such  dimensions  as  to  secure  their 
greatest  desirable  efficiency.  The  tripod  base  is  large  and 


J.  & W.  GRUNOW  & GO’S  ILLUSTRATED 


TIIE  student’s  larger  MICROSCOPE. 


29 


strong,  made  of  japanned  cast-iron,  giving  firm  support  and 
freedom  from  tremor.  The  coarse  adjustment  is  performed  by 
Fig.  10. 


NO.  4.  STUDENT’S  LARGER  MICROSCOPE. 
Fifteen  inches  high  when  arranged  for  use. 


a rack  and  pinion,  by  turning  a large  milled  head,  conven- 
iently placed ; the  body  moving  steadily  in  a long  grooved  sup- 
port, and  being  retained  in  any  position  by  springs.  The  fine 
adjustment  of  the  focus  is  performed  by  a screw  acting  upon  a 


CATALOGUE  OF  ACHROMATIC  MICROSCOPES. 


30 


MECHANICAL  PORTION  OF  THE  MICROSCOPE. 


lever,  which  gives  to  the  stage  a delicate  upward  movement. 
Two  sliding  clips  retain  the  object  on  the  stage. 

The  stage,  which  is  three  by  four  inches,  is  so  constructed 
that  it  can  be  moved  smoothly  and  steadily  in  every  direction, 
the  object  appearing  to  follow  the  motions  of  the  hand  upon 
the  lever.  This  movement  of  tlie  stage  gives  great  facility  for 
tracing  every  part  of  the  slide  in  the  search  for  delicate  ob- 
jects, and  enables  the  observer  to  follow  with  ease  the  motions 
of  living  animalculse,  even  with  high  powers.  Beneath  the 
stage  is  a circular  plate  carefully  centered  and  adapted  for  re- 
ceiving accessory  apparatus.  The  mirrors,  plane  and  concave, 
(the  latter  two  inches  in  diameter,)  are  so  mounted  as  to  have 
a free  and  steady  motion  in  every  direction.  By  means  of  an 
arm,  the  mirrors  can  be  thrown  far  out  from  the  axis  of  the 
microscope,  so  as  to  give  mvy  oblique  light  for  illuminating  the 
object. 

This  instrument  has  a graduated  draw-tube^  by  which  the  dis- 
tance between  the  objective  and  eye-piece  can  be  considerably 
increased.  This  increased  length  produces  a proportional  in- 
crease of  the  magnifying  power,  and  thus  often  greatly  aids 
in  ascertaining  the  value  of  micrometer  graduations.  (See 
71,  72.)* 

49.  Stage  Movable  by  Rack  and  Screiv.  American 
microscopists  generally  prefer  our  form  of  stage,  movable  by  a 
lever.  The  instruments  which  we  keep  on  hand  are,  therefore, 
usually  furnished  with  this  form  of  stage.  But  we  are  accus- 
tomed to  make  to  order  a stage  movable  in  two  rectangular 
directions,  by  rack  and  screw. 


* This  instrument  is  often  made  with  a plain  stage,  which  considerably  re- 
duces the  expense.  It  can  also  have  added  the  revolving  motion  of  the  stage, 
as  shown  in  the  next  figure.  Baileys’  Indicator  Stage  can  be  applied  to  this  in- 
strument, if  desired,  instead  of  the  stage  here  shown.  The  use  of  cast-iron,  for 
the  base  and  arm  of  the  preceding  instruments,  has  been  adopted  to  bring  the 
prices  within  the  most  reasonable  limits.  This  arrangement  does  not,  in  any  man- 
ner, diminish  the  efficiency  or  beauty  of  the  instruments.  The  parts  made  of  iron 
are  carefully  smoothed  and  neatly  japanned,  and  give  a pleasing  contrast  of  color 
with  the  other  parts,  which  are  of  brass.  But  when  specially  ordered  the  base 
and  arm  are  also  made  of  brass,  at  a reasonable  addition  to  the  price. 


J.  & W.  GRUNOW  & GO’S  ILLUSTRATED 


student’s  microscope. 


31 


50.  The  form  of  Stiidewt’s  Micro§copc,  shown  in  figure 
11,  lias  the  same  tripod  base,  mirrors,  and  graduated  draw-tube. 
Fig.  11. 


as  the  preceding,  and  the  same  lever  movement  of  the  stage. 
In  addition,  the  stage  revolves  around  a steady  centre,  which 
invariably  coincides  with  the  axis  of  the  body.  Beneath  the 


CATALOGUE  OF  ACHROMATIC  MICROSCOPES. 


32 


MECHANICAL  PORTION  OF  THE  MICROSCOPE. 


stage  is  a plate,  carefully  centered,  and  adapted  for  receiving 
achromatic  condenser,  and  other  apparatus.  A strong  trian- 
gular bar,  moved  by  a rack  and  pinion,  b}"  two  large  milled 
heads,  gives  the  quick  motion  of  the  body,  while  the  delicate 
fine  adjustment  is  effected  by  a screw  on  the  left  of  the  instru- 
ment, acting  upon  a lever,  which  gives  a slow  movement  to  the 
stage,  as  in  the  instrument  last  described. 

The  arm  which  carries  the  body  can  be  turned  away  from  over 
the  stage  ; this  is  a great  convenience  in  changing  the  object- 
glasses,  or  in  performing  any  manipulation  on  the  object  upon 
the  stage. 

The  body  of  the  instrument,  which  is  attached  by  a bayonet 
joint  to  the  arm  that  supports  it,  can  be  easily  removed,  for 
packing  in  a small  case,  and  the  instrument  can  be  readjusted 
for  use  with  great  facility.  When  the  body  is  removed,  this 
microscope  can  be  used  for  dissecting,  by  inserting  a single  lens 
into  the  end  of  the  movable  arm  in  place  of  the  body. 

Chevalier’s  Prismatic  Body,  shown  in  the  next  figure,  can 
be  attached  to  this  instrument. 

51.  Chevalier’s  Prismatic  Body,  shown  in  Fig.  12,  as  it 
is  attached  to  the  microscope,  consists  of  a tube  with  an  elbow 
in  which  is  inserted  a rectangular  reflecting  prism.  The  light 
from  the  object-glass  enters  the  prism  perpendicular  to  the  first 
surface,  and  falling  upon  the  second  surface  at  an  angle  of  45° 
sufl[ers  total  reflection,  and  emerges  perpendicular  to  the  third 
surface  of  the  prism,  which  makes  a right  angle  with  the  first. 
This  addition  is  particularly  useful  in  examining  living  animal- 
cules and  other  objects  in  water,  and  in  all  other  cases  where 
the  horizontal  position  of  the  stage  is  either  necessary  or  desira- 
ble ; and  when  otherwise  the  continued  posture  of  the  observ- 
er, in  looking  down  vertically,  would  be  attended  with  great 
fatigue  to  the  eye. 

This  apparatus  can  be  applied  to  microscopes  Nos.  5 and  G. 
A short  elbow  tube,  (containing  a rectangular  prism,)  one 
end  to  be  inserted  in  the  draw-tube  and  the  other  to  hold  the 
eye-piece,  is  furnished  to  order  with  any  of  our  microscopes. 
It  is  used  for  the  same  purpose  as  the  prismatic  body. 


J.  & W.  GRUNOW  & GO’S  ILLUSTRATED 


PROF,  bailey’s  indicator  STAGE. 


33 


Fig.  12. 


CHEVALIER’S  PRISMATIC  BODY,  AND  BAILEY’S  INDICATOR  STAGE. 

62.  Prof.  Bailey’s  Indicator  Stage,  shown  also  in  Fig.  12, 
as  attached  to  microscope  No.  5,  has  a graduated  movement  in 
two  directions,  by  turning  the  milled  heads  seen  under  the  stage. 
These  milled  heads  are  attached  to  two  pinions,  one  revolving 
within  the  other,  and  they  move  the  stage  in  two  directions  re- 
spectively at  right  angles  to  each  other.  On  each  border  of 
the  stage  are  scales  graduated  to  of  an  inch,  which  serve  to 
determine  the  exact  position  of  the  stage  when  an  object  is  in 
the  centre  of  the  field.  If  a slide  is  placed  in  a certain  fixed 
position  on  this  stage,  the  exact  position  of  any  object  seen  in 


CATALOGUE  OF  ACHROMATIC  MICROSCOPES. 

3 


34 


MECHANICAL  POETION  OF  THE  MICROSCOPE. 


the  centre  of  the  field  can  be  determined  by  the  graduation 
and  recorded  on  the  slide.  The  same  object  can  then  be  placed 
in  the  centre  of  the  field  of  view,  at  any  future  time  upon  any 
microscope  having  a similar  indicator. 

When  it  is  considered  that  with  the  higher  powers,  an  object 
measuring  only  j-Jo  or  5^  of  an  inch,  fills  the  entire  field  of 
view,  it  will  be  evident  that  objects  of  great  interest  are 
brought  into  the  field  of  view  a second  time  with  difiiculty, 
and  among  a number  of  similar  objects,  any  one  which  is 
peculiar  can  only  be  re-discovered  after  much  patient  research. 
But  with  this  indicator  stage,  if  the  position  of  an  object  has 
been  once  recorded,  it  can  be  found  again  with  great  ease  and 
certainty.  The  use  of  microscopes  furnished  with  this  stage 
afibrds  great  facility  for  interchanging  slides,  with  the  certain- 
ty of  finding  objects  of  interest  singled  out  by  correspondents. 

53.  The  Portable  Microscope,  shown  in  Fig.  13,  is  of 
the  same  size  as  FTo.  5,  but  more  portable. 

The  strong  brass  legs  on  which  it  is  mounted,  are  made  to 
fold  together,  and  the  compound  body,  which  is  attached  by  a 
bayonet  joint,  like  'No.  5,  can  be  easily  detached,  and  the  instru- 
ment packed  in  a very  small  case. 

The  arm  d,  which  carries  the  compound  body,  can  be  turned 
away  from  over  the  stage.  A stout  triangular  bar,  carrying 
the  arm  d,  is  moved  up  and  down  by  rack  and  pinion  connected 
with  two  milled  heads,  one  of  which  is  marked  c.  Tlius  either 
hand  may  be  used  for  the  quick  motion  of  the  body,  which  is 
effected  with  great  steadiness  and  freedom  from  tremor.  A 
delicate  fine  adjustment  of  focus  is  obtained  by  turning  the 
milled  head  a,  of  a screw  which  acts  upon  a lever  concealed  in 
the  arm  d ; this  lever  acts  upon  a short  tube,  carrying  the 
object-glass,  and  sliding  easily  but  firmly  in  the  lower  end  of 
the  body.  The  action  of  the  lever  upon  this  tube  is  counter- 
acted by  a spiral  spring,  so  that  an  extremely  sharp  and  deli- 
cate motion  is  obtained. 

The  elasticity  of  the  spring  diminishes  the  danger  of  injur- 
ing the  object-glass,  if  at  any  time  it  is  allowed  to  touch  the 
object.  Great  care  should  be  taken  that  no  such  accident 
should  ever  happen. 


J.  & W.  GRUNOW  & GO’S  ILLUSTRATED 


PORTABLE  MICROSCOPE. 


35 


Fig.  13. 


The  stage,  which  is  three  inches  square  and  provided  with 
sliding  clips,  is  moved  freely  in  every  direction  by  means  of 
the  lever  h / it  also  revolves  around  a steady  centre  coinciding 
with  the  axis  of  the  compound  body,  and  a circular  plate 


CATALOGUE  OF  ACHROMATIC  MICROSCOPES. 


MECHANICAL  PORTION  OF  THE  MICROSCOPE. 


beneath  the  stage  is  carefully  centered  and  fitted  to  receive  the 
achromatic  condenser  and  polarizing  apparatus. 

The  plane  and  concave  mirrors  (the  latter  inches  in 
diameter)  can  be  turned  freely  in  any  direction,  or  so  adjusted 
as  to  give  very  oblique  light.  The  instrument  has  a gradu- 
ated draw-tube,  and  the  whole  microscope,  with  all  the  acces- 
sory apparatus,  is  packed  in  a fiat  mahogany  case  of  very 
convenient  dimensions. 

54.  Tlie  Microscope  shown  in  Fig.  14,  is  of  the  largest  class, 
complete  in  all  its  parts,  and  constructed  upon  the  most  perfect 
model,  suggested  by  the  combined  experience  of  the  most 
eminent  American  microscopists. 

This  microscope  is  mounted  upon  a strong  brass  tripod,  with 
uprights  of  bell-metal  supporting  the  axis,  upon  which  the  in- 
strument can  be  inclined  at  any  angle.  The  stage  is  four 
inches  square,  movable  freely  in  every  direction  by  a lever  ; it 
also  revolves  around  a steady  centre,  coinciding  with  the  opti- 
cal axis  of  the  microscope. 

The  under  side  of  the  stage  is  fitted  for  the  attachment 
of  accessory  apparatus.  The  mirrors,  plane  and  concave,  (the 
latter  three  inches  in  diameter,)  are  mounted  with  a cradle 
joint,  and  movable  arm  attached  to  a sliding  support,  giving 
facility  of  movement  in  every  direction. 

The  compound  body  has  a graduated  draw-tube,  and  is 
attached  to  a heavy  socket  which  is  moved  up  and  down  on  a 
strong  triangular  bar  of  bell-metal,  by  rack  and  pinion,  by 
which  means  the  coarse  adjustment  is  efiected  with  great 
steadiness  and  freedom  from  tremor. 

The  fine  adjustment  of  focus  is  efiected  by  a screw  acting 
upon  a lever,  which  moves  a short  tube  carrying  the  object- 
glass.  This  tube  is  held  down  by  a spring,  which,  like  a 
similar  spring  in  FTo.  6,  diminishes  the  danger  of  injuring  the 
object-glass. 

This  instrument  is  finished  in  the  most  perfect  manner 
throughout,  and  is  designed  for  scientific  institutions,  and  for  all 
who  desire  to  possess  the  most  perfect  and  completely  furnished 
instruments. 


J.  k W.  GRUNOW  & CO’S  ILLUSTRATED 


LARGE  MICROSCOPE. 


37 


Fig.  14. 


Seventeen  inches  high  when  arranged  for  use. 


CATALOGUE  OF  ACHROMATIC  MICROSCOPES. 


38 


MECHANICAL  PORTION  OF  THE  MICROSCOPE. 


55.  Clieniical  or  Inverted  Microscopes.  The  chemical 
microscopist  frequently  has  occasion  to  perform  manipulations 
which  are  rendered  difficult  by  the  close  proximity  of  the 
object-glass  to  the  object,  or  to  apply  heat,  or  chemical  reagents, 
the  fumes  of  which  might  injure  the  lenses.  To  avoid  these 
difficulties,  Prof.  J.  Lawrence  Smith  has  invented  the  inverted 
microscope,  which  he  presented  to  the  Societe  de  Biologic  of 
Paris,  in  1850. 

In  this  microscope  the  object-glass  is  placed  below  the  stage, 
and  the  arrangement  of  the  several  parts  are  such  that  the  eye 
can  observe  the  object,  and,  almost,  at  the  same  time,  with  ease, 
guide  the  hands  in  performing  any  required  manipulation  on 
the  stage. 

The  optical  arrangement  of  the  inverted  microscope  is  shown 
in  Fig.  15. 


15.  E represents  the  stage 

with  the  object  upon  it, 
C is  the  object-glass  placed 
below  the  stage,  A is  a 
prism  so  constructed  that 
the  light  entering  it  at 
right  angles  to  the  face 
after  undergoing  total  re- 
flection at  h and  c,  emerges 
at  right  angles  to  the  face 
and  is  viewed  by  means 
of  the  eye-piece  placed  at 
D.  The  prism  A,  wdiich 
is  the  most  important  part 
of  the  instrument  of  this  form,  has  its  angles  ah^h  c d^  and 
d respectively  55°,  107 52^°,  and  145°,  consequently  the 
body  of  the  microscope  is  inclined  to  the  perpendicular  35°, 
which  is  found  to  be  the  most  convenient  position  for  steady 
and  protracted  observation.  The  illuminating  apparatus  for 
the  inverted  microscope  is  placed  above  the  stage,  and  consists 
of  a reflecting  prism  and  condensing  lenses. 


J.  & W.  GRUNOW  & GO’S  ILLUSTRATED 


THE  INVERTED  MICROSCOPE. 


39 


We  are  authorized  by  Prof.  Smith,  as  the  only  manufactur- 
ers of  these  instruments  in  this  country. 

56.  Tlic  simple  form  of  tlie  Inverted  Microscope,  shown 
in  Fig.  16,  is  designed  to  meet  the  ordinary  requirements  of  the 
chemist. 


Fig  16. 


NO.  8.  Prof.  J.  L.  SMITH’S  INVERTED  MICROSCOPE,  (Simp'e  Form.) 

This  instrument  has  coarse  and  fine  adjustments  of  focus, 
and  a round  stage  inches  in  diameter,  with  a glass  plate  for 
its  upper  surface,  and  spring  clips  to  retain  the  object  on  the 
stage.  It  will  be  seen  by  the  figure  that  the  rectangular  prism, 
used  as  a refiector,  and  the  condenser,  are  so  mounted  as 
to  be  moved  up  and  down  the  support,  or  inclined  at  any 
angle,  so  as  to  illuminate  the  object  with  very  oblique  light. 
The  tube,  which  forms  the  body  and  carries  the  eye-piece,  is 


CATALOGUE  OF  ACHROMATIC  MICROSCOPES. 


40 


MECHANICAL  PORTION  OF  THE  MICROSCOPE. 


attached  by  a bayonet  joint,  and  can  be  removed  to  allow  of 
packing  in  a convenient  case.  Polarizing  apparatus  and  other 
accessories,  are  furnished  to  order  with  this  instrument. 

57.  A more  complete  Inverted  HIicro§cope  is  mounted 
on  a revolving  foot,  has  coarse  and  fine  adjustments  of  focus, 
a stage  movable  by  a lever,  and  a column  with  rack  and  pinion 
movement  to  carry  either  the  illuminator,  polarizing  apparatus, 
or  oblique  condenser.  It  can  be  furnished  with  all  the  accesso- 
ries supplied  with  other  instruments. 

The  especial  use  of  the  inverted  microscope  is  for  investigation 
of  chemical  substances,  but  it  also  afibrds  advantages  in  exam- 
ining all  objects  contained  in  fluids,  for  however  deep  the  cell, 
the  object  lying  at  the  bottom  is  seen  as  though  mounted  in  the 
shallowest  cell.  By  this  instrument  small  insects,  animalculse 
and  infusorise,  as  Desmidise  and  Diatomaceae  are  viewed  from 
their  under  surface,  which  often  materially  aids  in  investigating 
their  structure. 


58.  OBJECT-OliASSE§.  Our  achromatic  object-glasses 
range  from  2 inch  to  ^^2  i^ch  focus,  with  magnifying  powers 
varying  from  20  to  over  1600  diameters,  which  may  be  in- 
creased, by  extending  the  draw-tube  of  the  microscope,  to  2000 
diameters.  A table  containing  a list  of  our  objectives,  with 
the  angular  aperture  and  magnifying  power  of  each,  with 
the  different  eye-pieces,  will  be  found  in  connection  with  the 
Price  List  at  the  end  of  this  Catalogue. 

To  suit  the  requirements  of  different  observers,  we  furnish  two 
classes  of  achromatic  objectives.  Our  objectives  of  the  First 
Class  have  very  large  angular  aperture,  with  the  most  perfect  cor- 
rection of  spherical  and  chromatic  aberration,  and  are  mounted 
in  the  best  manner,  and  in  the  most  improved  style,  and  are  at- 
tached to  the  microscope  with  a bayonet  join t."^  The-|,  J,  J,  and 
^ inch  objectives  of  this  class  are  furnished  with  Mr.  Wenham’s 
form  of  adjustment  for  correcting  the  aberration  produced  by 

* Our  objectives  will  be  attached  to  the  microscope  by  a screw,  for  those  who 
prefer  that  mode  of  attachment. 


J.  & W.  GRUNOW  & GO’S  ILLUSTRATED 


MOUNTING  OF  OBJECT-GLASSES. 


41 


the  tliin  glass  which  covers  the  object.  The  one  inch  and  two 
inch  objectives  do  not  require  this  adjustment. 

Figure  17  shows  the  method  of  mounting  the  best  object- 
glasses,  with  Mr.  Wen- 
ham’s  adjustment  for 
the  thickness  of  glass 
cover,  A being  a longi- 
tudinal section,  and  B 
an  exterior  view  of  the 
same. 

The  anterior  com- 
pound lens  is  perma- 
nently fixed  to  the  out- 
er tude,  or  that  part  of 
the  mounting  which 
connects  the  objective 
to  the  body  of  the  microscope,  while  the  middle  and  posterior 
combinations  are  set  in  a tube  which  slides  wuthin  the  other. 
A revolving  collar,  with  its  milled  edge  5,  is  attached  by  a 
screw  to  the  inner  tube,  the  screw  passing  through  a slide  in  an 
inclined  slit  in  the  tube  which  carries  the  anterior  combination. 
When  the  collar  is  revolved,  the  screw  and  slide  moving  in  the 
inclined  slit,  the  inner  tube,  with  the  posterior  and  middle  com- 
pound lenses,  is  made  to  approach  or  recede  from  the  anterior 
combination. 

The  slit  in  the  outer  tube  is  so  much  inclined  that  a quarter 
of  a revolution  gives  to  the  inner  tube  the  greatest  extent  of 
movement  that  is  ever  required.  This  movement  of  the  middle 
and  posterior  combinations  effects  the  adjustment  required  by 
covering  the  object  with  glass  of  any  required  thickness.  The 
point  on  the  collar  marked  uncovered'^^  is  the  point  of  adjust- 
ment which  gives  the  most  perfect  definition  of  an  uncovered 
object;  the  part  marked  ‘Hhin  glass’^  indicates  the  correction 
for  glass  of  medium  thickness.  Between  these  two  points  are 
ten  or  more  divisions,  and  several  equal  spaces  are  marked  oft* 
beyond  this  point. 

This  graduation  is  exceedingly  convenient  for  many  purposes, 


CATALOGUE  OF  ACHROMATIC  MICROSCOPES. 


42 


IVIECIIANICAL  PORTION  OF  THE  MICROSCOPE. 


especiall}^  to  facilitate  the  adjustment  of  the  draw-tube,  when 
using  the  micrometer. 

59.  Rules  for  Adjusting:  Object-Olasses.  As  many  per- 
sons who  use  the  miscroscope  find  some  embarrassment  in  accu- 
rately adjusting  the  object-glass  for  the  thickness  of  the  cover 
on  different  objects,  and  hence  fail  to  appreciate  the  real  excel- 
lence of  the  instrument,  we  subjoin  a few  simple  rules,  by 
means  of  which  any  one  may  soon  learn  to  make  this  cor- 
rection. 

Select  any  dark  speck  or  opaque  portion  of  the  object,  and 
bring  the  outline  into  perfect  focus ; then  lay  the  finger  on  the 
milled  head  of  the  fine  motion,  and  move  it  briskly  backwards 
and  forwards  in  both  directions  from  the  first  position.  Ob- 
serve the  expansion  of  the  dark  outline  of  the  object,  both 
when  within  and  when  without  the  focus.  If  the  greater  ex- 
pansion, or  coma,  is  when  the  object  is  within  the  focus,  or 
nearest  to  the  objective,  the  lenses  must  be  placed  farther 
apart,  or  towards  the  mark  uncovered P If  the  greater 
coma  is  when  the  object  is  without  the  focus,  or  farthest  from 
the  objective,  the  lenses  must  be  brought  closer  together,  or  to- 
wards the  mark  ‘Hhin  glassP  When  the  object-glass  is  in 
proper  adjustment,  the  expansion  of  the  outline  is  exactly  the 
same  both  within  and  without  the  focus. 

A different  indication,  however,  is  afforded  by  such  test  ob- 
jects as  present  (like  the  Podura  scale  and  Diatomaceje)  a set 
of  distinct  dots  or  other  markings.  For  if  the  dots  have  a 
tendency  to  run  into  lines  when  the  object  is  placed  within 
the  focus,  the  glasses  must  be  brought  closer  together  ; on  the 
contrary,  if  the  lines  appear  when  the  object  is  without  the 
focal  point,  the  combinations  of  the  object-glass  must  be  fur- 
ther separated. 

60.  Delicacy  of  tlie  Adjii§tmciit  for  Tliiot  Covers. 

When  the  angle  of  aperture  is  very  wide,  the  difference  in 
the  aspect  of  any  severe  test,  under  different  adjustments, 
becomes  at  once  evident;  markings  which  are  very  distinct 
when  the  correction  has  been  exactly  made,  disappearing  al- 
most instantaneously  when  the  graduated  collar  has  been  turned 


J.  & W.  GRUNOW  & CO’S  ILLUSTRATED 


DELICACY  OF  THE  ADJUSTMENT  FOR  THIN  COVERS. 


43 


through  no  more  than  half  a division  of  its  graduated  scale. 
With  this  form  of  adjustment,  the  transition  from  good  to  bad 
definition  takes  place  with  such  a slight  movement  of  the  col- 
lar, that  with  a little  practice,  the  best  point  of  definition  is 
readily  found. 

It  often  happens  that  the  amount  of  balsam,  or  other  me- 
dium, covering  different  objects  on  the  same  slide,  varies,  there- 
fore, the  adjustment  should  be  examined  for  each  portion  of  the 
slide,  where  we  wish  to  make  the  most  accurate  observa- 
tions. 

For  object  glasses  of  the  largest  angular  aperture,  very  great 
care  and  patience  are  often  required  in  effecting  the  most  per- 
fect adjustment.  With  glasses  of  moderate  aperture,  if  the 
adjustment  has  been  well  made  for  glass  cover  of  medium 
thickness,  the  same  correction  will  answer  very  well  for  ordi- 
nary observations,  even  should  the  thickness  of  the  glass  cover 
be  slightly  varied.  For  common  investigations,  therefore, 
much  time  may  be  gained  by  assorting  the  thin  glass  into  par- 
cels of  nearly  uniform  thickness,  and  having  obtained  a medi- 
um adjustment  for  one  variety,  so  long  as  the  same  parcel  of 
thin  glass  is  used,  the  labor  of  adjusting  for  thickness  of  glass 
cover  may  be  dispensed  with.  But  when  glass  of  another 
thickness  is  employed,  or  when  a more  delicate  object  is  to  be 
examined,  or  an  objective  of  very  large  aperture  is  employed, 
the  adjustment  for  thickness  of  cover  must  be  examined  and 
made  as  accurate  as  possible. 

Increasing  the  distance  between  the  objective  and  eye-piece, 
by  extending  the  draw-tube,  causes  disturbance  of  the  balance 
of  aberrations,  and  requires  renewed  adjustment  by  moving 
the  graduated  collar,  in  the  same  manner  as  for  different  thick- 
nesses of  glass  cover. 

When  an  object-glass  is  examined  that  has  no  correction  for 
thickness  of  glass  cover,  it  will  usually  be  found  that  it  has  but 
very  moderate  angular  aperture.  Such  is  the  case  with  almost 
all  object-glasses  made  on  the  continent  of  Europe. 

61.  Our  §econd  Class  Objectives  have  a somewhat  smaller 
angular  aperture,  and  are  mounted  in  a simpler  style.  They 


CATALOGUE  OF  ACHROMATIC  MICROSCOPES. 


u 


MECHANICAL  PORTION  OF  THE  MICROSCOPE. 


are  attached  by  a screw  to  an  adapter,  which  is  inserted  by  a 
bayonet  joint  into  the  body  of  the  microscope.  They  are  very 
carefully  corrected  for  chromatic  and  spherical  aberration,  and 
are  adjusted  for  a glass  cover  xio  of  an  inch  thick.  With  this 
correction,  which  is  permanently  fixed,  this  class  of  objectives 
will  perform  very  nearly  as  well  if  the  thickness  of  glass  cover- 
ing an  object  is  one-eightieth  or  one  hundred  and  twentieth  of 
an  inch  in  thickness.  These  objectives  will  resolve  nearly  all 
the  test  objects  mentioned  in  works  on  the  microscope.  The 
i inch  of  this  class,  with  eye-piece  ITo.  2,  with  good  illumina- 
tion, will  show  very  clearly  all  the  lines  and  dots  on  the  JPleu- 
rosigma  angulata.  The  inch  will  resolve  the  Namcula 
Baltica  and  Hijppocamjpus  in  balsam,  and  will  resolve  the 
PleuTOsigma  angulata  when  mounted  dry.  All  this  class  of 
objectives  answer  well  for  investigations  in  botany,  entomology, 
and  pathology.  The  low  price  at  which  this  class  of  object- 
ives is  furnished,  will  commend  them  to  all  who  desire  cheap, 
but  good  object-glasses. 


J.  & W.  GRUNOW  & GO’S  ILLUSTRATED 


THE  COBWEB  MICROMETER. 


45 


CHAPTER  III. 

ACCESSORY  APPARATUS. 

62.  Micrometers.  In  examiningjobjects  with  the  micro- 
scope, it  is  often  desirable  to  ascertain  their  exact  dimensions. 
Measuring  instruments  for  this  purpose,  called  micrometers, 
are  sometimes  applied  to  the  object  itself,  but  more  frequently 
to  the  magnified  image.  Both  classes  of  micrometers  have 
their  uses,  for  which  they  are  specially  adapted,  and  the  pur- 
chaser can  select  that  form  which  is  best  suited  to  the  investi- 
gations for  which  it  is  to  be  used. 

63.  Glass  Stage  Micrometers  are  furnished  mounted  in 
brass,  with  lines  ruled  from  fo  x^o  of  an  inch.  This  instru- 
ment is  chiefiy  used  for  determining  the  value  of  measurements 
made  by  the  various  eye-piece  micrometers. 

64.  Tlie  Cobweb  Micrometer  was  invented  by  Rams- 
den  for  telescopes,  but  it  is  equally  applicable  to  the  micro- 
scope, and,  until  lately,  has  been  considered  superior  to  all 
others.  Ramsden  employed  the  positive  eye-piece,  but  the 
negative  eye-piece  is  equally  available,  and  for  use  with  the 
microscope  it  is  to  be  preferred. 

The  Cobweb  micrometer  is  shown  in  Figs.  18  and  19,  and 
consists  of  a negative  eye-piece,  in  the  focus  of  which  two 
cobweb  threads  are  stretched  across  the  field ; one  of  these 
threads  can  be  separated  from  the  other  by  a screw  having 
about  fifty  threads  to  the  inch.  The  head  of  this  screw,  shown 
at  Fig.  18,  is  divided  into  one  hundred  parts. 


CATALOGUE  OF  ACHROMATIC  MICROSCOPES. 


46 


ACCESSORY  APPARATUS. 


Fig.  18. 


Fig.  19. 


A portion  of  the  field  of  view  on  one  side  is  cut  off  at  right 
angles  to  the  cobweb  threads,  Fig.  19,  by  a scale  formed  of  a thin 
plate  of  brass  having  notches  at  its  edge,  whose  distance  cor- 
responds to  that  of  the  threads  of  the  screw,  every  fifth  notch 
being  made  deeper  than  the  rest  for  the  sake  of  ready  enumer- 
ation. As  each  notch  corresponds  to  one  turn  of  the  screw,  the 
number  of  turns  can  be  read  off  in  the  field  of  the  instrument, 
and  the  fraction  of  a turn  on  the  graduated  head. 

By  this  simple  contrivance,  the  distance  of  the  threads  can  be 


J.  & W.  GRUNOW  & GO’S  ILLUSTRATED 


JACKSON  S MICROMETER. 


47 


ascertained  to  the  hundredth  of  a turn  of  the  screw,  and  as  the 
screw  has  fifty  threads  to  an  inch,  it  follows  that  the  magnified 
image  of  an  object  may  be  measured  to  the  five-thousandth  of 
an  inch. 

With  an  object-glass  of  one-eighth  of  an  inch  focus,  the  im- 
age, formed  at  the  focus  of  the  negative  eye-piece,  where  the  mi- 
crometer threads  are  placed,  will  be  magnified  about  fifty  diam- 
eters without  the  power  of  the  eye-lens ; it  follows,  therefore, 
that  a quantity  as  small  as  the  two-hundred-and-fifty-thousandth 
of  an  inch  should  be  appreciable  by  such  an  instrument,  but 
in  practice  this  has  been  found  impossible,  as  no  achromatic 
power  has  yet  been  made,  capable  of  separating  lines  so 
close  as  the  one-hundred-thousandth  of  an  inch.  Great  care  is 
requisite  in  ascertaining  the  value  of  the  measurements  made 
by  this  micrometer,  to  avoid  several  species  of  error  which 
will  be  pointed  out  in  section  71.  The  measurements  by  this 
micrometer  are  not  as  delicate  as  they  appear  to  be,  but  are 
the  most  reliable  that  can  be  obtained  by  any  eye-piece  mi- 
crometer. 

65.  Rosses  Eye-Piece  Micrometer  consists  of  a circle  of 
thin  glass,  ruled  with  micrometer  lines,  set  in  a brass  ring,  and 
screwed  into  the  lower  end  of  the  positive  eye-piece,  so  as  to  be 
seen  exactly  in  its  focus.  To  adj  ust  the  focus  to  suit  difierent 
eyes,  the  ring  may  be  screwed  up  a little  nearer  to  the  front 
lens,  or  adjusted  a little  more  distant.  This  is  a very  conven- 
ient form  of  micrometer  for  use  with  low  powers. 

66.  Jackson’s  Micrometer.  Mr.  George  Jackson,  in  1840, 
invented  another  form  of  micrometer,  which,  with  the  improve- 
ments suggested  by  experience,  he  thus  describes : 

Short  bold  lines  are  ruled  on  a piece  of  glass;  and,  to  facil- 
itate counting,  the  fifth  is  drawn  longer,  and  the  tenth  still  long- 
er, as  in  the  common  rule.  Yery  finely  levigated  plumbago  is 
rubbed  into  the  lines  to  render  them  visible,  and  they  are  cov- 
ered with  a piece  of  thin  glass,  cemented  by  Canada  balsam^ 
to  secure  the  plumbago  from  being  rubbed  out.” 

The  slip  of  glass  thus  prepared  is  placed  in  a thin  brass  frame, 
as  shown  in  Fig.  20,  so  that  it  may  slide  freely,  and  is  acted  on 
at  one  end  by  a pushing  screw,  and  at  the  other  by  a slight 


CATALOGUE  OF  ACHROMATIC  MICROSCOPES. 


48 


ACCESSORY  APPARATUS. 


spring.  This  is  inserted  in  the  focus  of  the  eje-lens  in  the  neg- 
ative eje-piece,  through  slits  cut  in  each  side  for  the  purpose. 
The  cell  of  the  eye  glass  should  have  a longer  screw  than  usual 
to  admit  adjustment  for  different  eyes.  When  the  frame  is  not 
employed,  an  inner  piece  of  tube  is  drawn  across  the  slits  to  pre- 
vent dust  from  getting  between  the  glasses. 

Fig.  20. 


To  use  this  micrometer,  the  object  is  brought  to  the  centre 
of  the  field,  and  the  coincidence  between  one  side  of  it,  and  one 
of  the  long  lines,  is  made  with  great  accuracy,  by  means  of  the 
small  pushing  screw  that  moves  the  slip  of  glass  ; the  divisions 
are  then  read  off  as  easily  as  the  inches  and  tenths  on  a com- 
mon rule.  The  value  of  the  divisions  must,  however,  be  accu- 
rately ascertained  for  each  object-glass,  in  the  same  manner  as 
for  other  eye-piece  micrometers.  The  operation  of  measure- 
ment is  then  nothing  more  than  laying  a rule  across  the  body  to 
be  measured  ; and  it  matters  not  whether  the  object  be  trans- 
parent or  opaque,  mounted  or  not  mounted  ; if  its  edges  can 
be  distinctly  seen,  its  diameter  can  be  taken.  By  revolving 
the  eye-piece,  similar  measurements  can  be  taken  across  any 
other  diameter. 

A similar  micrometer  slide,  with  its  pushing  screw,  is  attach- 
ed to  the  positive  eye-piece,  when  this  is  preferred. 

67.  Comparative  Merits  of  Micrometers.  Of  these  mi- 
crometers, the  one  shown  in  figures  18  and  19  is  the  most  accu- 
rate and  reliable.  Its  measurements  are  all  made  between  two 
lines,  neither  of  which  is  included  in  the  measurement,  the  clear 
space  between  the  lines  being  alone  reckoned.  As  the  neg- 
ative eye-piece  is  used,  the  object  is  clearly  defined,  free  from 
all  aberration.  The  expense  of  the  Cobweb  Micrometer  alone 
prevents  its  general  use. 

In  Mr.  Boss’s  micrometer,  the  use  of  the  positive  eye-piece 


J.  & W.  GRUNOW  & GO’S  ILLUSTRATED 


DR.  WHITE  S MICROMETER. 


40 


somewhat  impairs  the  definition  of  the  object.  The  definition 
is  still  farther  impaired  by  the  glass  on  which  the  lines  are 
ruled,  which  covers  the  entire  field  of  view.  The  lines  them- 
selves exhibit  a sensible  breadth,  and  the  exact  j uxtaposition  of 
the  lines  and  object  are  not  so  easily  secured,  yet  for  low  pow- 
ers this  micrometer  answers  a good  purpose. 

AYith  Mr.  Jackson’s  micrometer  and  the  negative  eye-piece, 
more  accurate  results  are  obtained,  and  although  the  fact  that 
the  micrometer  glass  covers  the  whole  field,  impairs  its  useful- 
ness with  high  powers,  yet  its  measurements  are  as  accurate  as 
are  required  for  ordinary  observations. 

68.  I>r.  White’s  Micrometer.  At  the  suggestion  of  Dr. 
White,  of  this  city,  we  have  recently  made  micrometers  of  an- 
other very  simple  form,  which  have  given  great  satisfaction. 
Fig.  21  represents  one  of  these  micrometers. 

Fig.  21. 


A semi-circular  piece  of  glass,  A C B,  has  micrometer  lines 
ruled  as  shown  in  the  figure.  This  is  cemented  to  the  dia- 
phragm of  the  negative  eye-piece,  and  occupies  very  nearly 
one-half  of  the  field.  The  edge  of  the  glass  appears  as  a dark 
line  A B,  across  the  field,  while  A D B,  occupying  a very  little 
more  than  one-half  the  field,  is  entirely  unobstructed,  as  though 
no  micrometer  were  used.  This  allows  the  eye-piece  contain- 
ing this  micrometer  to  be  used  for  ordinary  observations.  By 


CATALOGUE  OF  ACHROMATIC  MICROSCOPES. 
4 


50 


ACCESSORY  APPARATUS. 


moving  the  object,  or  rotating  the  eye-piece,  the  lines  can  easily 
be  brought  to  measure  any  object  in  the  field.  The  object  may 
be  brought  up  to  the  ends  of  the  lines  without  being  covered 
by  the  micrometer  glass. 

With  this  micrometer,  in  the  eye-piece  used  for  ordinary  ob- 
servation, hundreds  of  objects  may  be  measured,  whose  magni- 
tude would  pass  unnoted  if  it  were  necessary  to  change  the  eye- 
piece to  effect  the  measurement.  These  advantages,  combined 
with  the  very  low  price  for  which  it  is  furnished,  will,  we  think, 
cause  this  micrometer  to  be  regarded  with  very  general  favor, 
and  observers  who  have  more  expensive  micrometers,  will  find 
it  convenient  to  possess  this  also.  The  lines  in  this  micrometer 
are  ruled  to  or  5^0  of  an  inch,  to  suit  purchasers,  but  the 
value  of  the  measurements  made  by  them  must  be  calculated 
for  each  object-glass,  in  the  same  manner  as  with  other  eye- 
piece micrometers. 

69.  Prof.  J.  li.  Smitli’s  Ooiiioineter  and  Micrometer. 


Fig.  22. 


Prof.  Smith  has  invented  a Goniometer  for  measuring  the 
angles  of  crystals  under  the  microscope.  It  is  also  combined 
with  a micrometer.  The  following  description  of  the  instru- 
ment and  the  method  of  using  it,  are  taken  from  the  Appendix 
to  Carpenter  on  the  Microscope^  edited  by  F.  G.  Smith,  M.  D., 
American  edition.  Philadelphia : Blanchard  & Lea.  1856. 

E,  Fig.  22,  is  the  upper  end  of  the  draw-tube  of  the  micro- 
scope with  the  ring  Ic  soldered  to  it.  Over  this  ring  screws 

another  ring  F,  which 
serves  as  a support  and  a 
centre  to  the  graduated 
circle  D,  which  freely,  but 
without  shaking,  revolves 
upon  the  same.  Into  the 
bore  of  the  ring  F fits  by  its 
lower  conical  end  A,  tlie 
tube  G,  which  is  held  in  it 
by  the  screw-ring  c>,  that 
prevents  its  being  taken 
out.  Into  the  tube  G, 
which  also  has  a free  re- 


J.  & W.  GRUNOW  & CO’S  ILLUSTRATED 


GONIOMETER  AND  MICROMETER. 


51 


volving  movement,  fits  rig-  23. 

the  positive  eye-piece 
d being  the  field-lens, 
and  8 the  eye-lens. 

The  slits  h h,  on  oppo- 
site sides  of  G,  (the  ref- 
erences in  Figs.  22  and 
23  are  the  same,)  allow 
the  micrometer  with  its 
mounting  B B,  to  be  in- 
troduced into  G,  and 
permit  the  graduated 
lines  to  be  brought  into 
the  field  of  the  eye-piece. 

C is  an  index,  attached 
to  G by  the  screw  c,  it  may  be  taken  off  when  the  apparatus 
is  not  used  as  a goniometer. 

70.  Method  of  using  the  Ooiiiometer.  Bring  the  object 
into  focus,  near  the  centre  of  the  field  of  the  micrometer, 
applying  the  finger  to  the  knob  K,  revolve  the  micrometer  till 
the  lines  of  its  graduation  are  parallel  to  one  side  of  the  angle 
to  be  measured.  Bevolve  then,  separately,  the  graduated 
circle  till  zero  is  brought  to  agree  with  the  point  of  the  index 
C.  Then  revolve  again  the  micrometer  by  the  knob  K,  until 
the  graduation  lines  are  parallel  to  the  other  side  of  the  angle 
to  be  measured,  when  the  index  C will  show  the  value  of  this 
angle. 

The  micrometer  lines  are  about  iiich  apart,  but 

their  value,  when  used  for  measurements  with  the  different 
object-glasses  and  eye-pieces,  must  be  ascertained  by  a stage 
micrometer  and  recorded  in  a table. 

71.  Method  of  finding  the  value  of  lines  in  any  Eye- 
piece Micrometer.  For  this  purpose  we  must  employ  a stage 
micrometer,  having  lines  ruled  at  some  known  distance,  and 
this  instrument  should  be  of  the  very  best  quality,  as  the  accu- 
racy of  all  our  measurements  wdth  the  eye-piece  micrometer 
depend  on  the  accuracy  of  the  instrument  with  which  their 
values  are  determined. 


CATALOGUE  OF  ACHROMATIC  MICROSCOPES. 


0,  OF  [LL'  UB« 


52 


ACCESSORY  APPARATUS. 


To  illustrate  the  method  of  making  these  calculations,  we 
give  the  process  and  results  in  a single  case,  using  the  ■§■  inch 
objective  and  negative  eye-piece  [N’o.  2,  in  which  is  inserted  a 
glass  micrometer,  with  lines  ruled  about  2 Jo  inch;  (the 

exact  value  of  these  lines  is  of  no  conse(][uence,  as  their  value, 
as  used  for  micrometer  measurements,  depends  on  the  magnify- 
ing power  of  the  glasses  used.)  We  select  a stage  micrometer 
with  lines  ruled  at  of  an  inch,  with  no  covering  over  the 
lines.  We  place  this  micrometer  on  the  stage  with  the  ruled 
lines  upward.  Setting  the  adjustment  of  the  object-glass  at  the 
mark  ^ uncovered^  and  carefully  adjusting  the  focus,  we  find 
one  space  on  the  stage  micrometer  covers  very  nearly  nineteen 
spaces  in  the  eye-piece  micrometer  ; we  therefore  increase  the 
magnifying  power,  by  extending  the  draw-tube,  till  a conven- 
ient number,  as  twenty  spaces,  are  covered  by  one  space  on  the 
stage,  when  we  find  that  the  draw-tube  is  extended  xVo 
inch. 

As  now  twenty  spaces  in  the  eye-piece  micrometer  are  equal 
to  5 Jo  an  inch  on  the  stage,  each  division  in  the  eye-piece 
measures  toJw  of  an  inch  on  the  stage,  when  the  object  is 
uncovered,  and  the  draw-tube  is  extended  of  an  inch.  We 
find  this  estimate  exceedingly  convenient  for  use,  and  accord- 
ingly record  the  conditions  and  estimated  measurement  for 
future  reference. 

But  as  our  measurement  will  often,  perhaps  generally,  be 
made  upon  objects  covered  with  thin  glass,  we  now  place 
a cover  of  thin  glass  over  the  micrometer  lines  on  the  stage, 
and  repeat  our  calculations.  We  correct  the  object-glass  for 
thickness  of  glass  cover,  by  turning  the  graduated  collar  till  we 
obtain  perfect  definition  of  the  lines  on  the  stage  micrometer, 
and  we  find,  in  this  case,  that  the  adjustment  of  the  object- 
glass  has  been  turned  forward  twelve  and  one-half  degrees  of 
its  graduated  scale ; counting  the  spaces  in  the  eye-piece 
micrometer,  covered  by  one  space  on  the  stage,  we  find,  not 
twenty  as  before,  but  twenty-one  and  a fraction;  we  therefore 
diminish  the  magnifying  power  by  pushing  back  the  draw-tube 
till  twenty  spaces  in  the  eye-piece  again  exactly  fill  one  division 


J.  & W.  GRUNOW  & GO’S  ILLUSTRATED 


METHOD  OF  USING  THE  MICROMETER. 


53 


in  the  stage  micrometer,  and  we  find  the  draw-tube  extended 
only  to  inches.  Thus,  in  covering  the  object  with  thin 
glass,  and  turning  forward  the  adjustment  of  the  object-glass 
twelve  and  one-half  degrees,  we  have  found  it  necessary  to  push 
in  the  draw-tube  jVo  inch,  or  yf  □ inch  for  every  degree  the 
adjustment  of  the  object-glass  has  been  turned  forward. 

The  same  method  is  to  be  pursued  in  estimating  the  value  of 
the  micrometer  lines  for  every  object-glass  with  which  it  is  to 
be  used.  The  values  here  given  apply  only  to  the  particular 
instrument  and  glasses  used  in  this  calculation,  for  slight  differ- 
ences in  glasses  of  the  same  name,  and  different  lengths  of  the 
compound  body,  however  slight,  cause  the  measurements  of  the 
micrometer  to  vary.  Hence  every  microscope  should  have  its 
own  table  of  measurements  for  its  micrometer. 

72.  Method  of  using  the  Micrometer.  Suppose  we  are 
examining  a delicate  Diatomacea  with  the  instrument  which 
we  have  used  in  making  the  preceding  calculations,  and  we 
wish  to  measure  the  breadth  of  the  rows  of  hexagonal  mark- 
ings which  we  find  covering  the  object.  We  examine  the 
adjustment  of  the  object-glass,  and  find  that  it  stands  at  eleven 
degrees,  then  yf ^ inch  for  each  degree  would  give  y^^  of  an 
inch,  which  the  present  adjustment  requires  to  be  subtracted 
from  yW»  the  distance  the  draw-tube  is  to  be  extended  for  an 
uncovered  object,  which  leaves  yo”^  of  an  inch  as  the  distance 
the  draw-tube  is  to  be  extended  to  have  our  micrometer  lines 
measure  ten-thousandths  of  an  inch  ; adjusting  the  draw-tube 
to  this  calculation,  we  see  that  two  spaces  in  the  micrometer 
cover  nine  rows  of  the  delicate  hexagonal  markings  on  our 
object,  {navicula  angulata^)  therefore  the  breadth  of  each  row 
is  Tsioo  of  an  inch. 

Suppose  any  object  measured  is  equal  to  a certain  number  of 
spaces  and  a small  part  of  an  additional  space, one-half,  or  even 
a quarter  of  a space  between  two  micrometer  lines  can  gen- 
erally be  estimated  with  tolerable  accuracy,  so  that  the  meas- 
urements made  by  the  glass  micrometers  attached  to  a nega- 
tive eye-piece,  may  be  relied  upon  for  measurements  as  small 
as  one  twenty  thousandth,  or  even  one  forty  thousandth  of  an 


CATALOGUE  OF  ACHROMATIC  MICROSCOPES. 


54 


ACCESSORY  APPARATUS. 


inch  ; which  is  a degree  of  accuracy  sufficient  for  all  ordinary 
observations.  Those  who  desire  more  accurate  measurements 
will,  of  course,  procure  the  more  expensive  instruments.  For 
more  detailed  descriptions  of  the  method  of  using  microme- 
ters, we  would  refer  our  readers  to  the  valuable  works  of 
Quecket  and  Carpenter  on  the  microscope. 

73.  Frainihofer’s  Stage-§crew-Micro]iieter.  The  labor 
of  making  the  necessary  calculations  to  determine  the  value  of 
measurements  made  by  micrometers  placed  in  the  eye-piece  of 
the  microscope,  and  the  amount  of  care  required  to  secure 
accurate  results,  have  rendered  it  desirable  to  obtain  a microm- 
eter at  once  minutely  accurate,  simple  in  its  use,  and  requiring 
no  calculation  of  the  value  of  its  measurements. 

The  instrument  most  nearly  fulfilling  these  requirements 
is  Fraunhofer’s  Stage-Screw-Micrometer.  The  value  of  the 
measurements  made  by  this  micrometer  is  always  independent 
of  the  magnifying  power  of  the  microscope.  The  instrument 
indicates,  directly,  and  with  great  accuracy,  the  absolute 
dimensions  of  the  object  measured. 

This  micrometer  is  placed  upon  the  stage  and  consists  essen- 
tially of  two  plates,  one  of  which,  carrying  the  object,  is 
moved  upon  the  other  by  a micrometer-screw,  having  one 
hundred  threads  to  an  inch,  with  a graduated  head  and  vernier 
by  which  motions  of  the  plates  and  object  upon  it  are  accu- 
rately measured  to  the  one  hundred  thousandth  of  an  inch. 

Fig.  24  shows  this  instrument,  the  upper  part  of  the  figure 
being  a view  from  above,  while  the  lower  part  of  the  figure 
gives  a side  view  of  the  same. 

B B is  the  body  of  the  instrument  which  is  fastened  to  the 
stage  of  the  microscope  by  the  short  cylinder  C,  which  is  made 
elastic  so  as  to  hold  the  instrument  securely  in  its  place  when 
it  is  inserted  into  the  circular  opening  of  the  stage.  A A 
is  a revolving  plate  with  spring  clips  to  support  the  object  to 
be  measured.  This  plate  can  be  revolved  so  as  to  bring  the 
object  in  a position  to  measure  it  in  any  direction.  This  circu- 
lar plate  is  supported  on  another  plate  in  the  body  of  the 
instrument,  which  is  moved  by  a fine  micrometer  screw,  with 
exactly  one  hundred  threads  to  the  American  standard  inch. 


.J.  & W.  GRUNOW  & GO’S  ILLUSTRATED 


Fraunhofer’s  stage-screw-micrometer. 


55 


Fig.  24. 


On  the  end  of  this  screw  is  a milled  head  m,  with  a head 
having  one  hundred  divisions  ; by  means  of  the  vernier  ti,  each 
division  of  the  scale  g is  divided  into  ten  parts,  each  of  which 
is  equivalent  to  one  thousandth  part  of  a turn  of  the  screw. 
The  graduated  head  g can  be  turned  without  moving  the  milled 
head  m or  the  screw  to  which  it  is  attached,  by  which  means 
the  graduation  can  be  set  at  zero  in  any  position  of  the  screw 
and  object.  At  -y  is  a scale  showing  the  number  of  turns 
given  to  the  screw,  each  equal  to  one  hundredth  of  an  inch, 
therefore  the  divisions  of  g are  ten  thousandths,  and  the  divi- 
sions shown  by  the  vernier  are  hundred  thousandths  of  an  inch, 
whatever  may  be  the  magnifying  power  of  the  microscope 
by  which  the  object  is  viewed. 

Accompanying  this  micrometer  is  a negative  eye-piece  with 
a cobweb  drawn  across  the  field  of  view,  in  the  focus  of 
the  eye-lens,  the  magnitude  of  an  object  being  determined 


CATALOGUE  OF  ACHROMATIC  MICROSCOPES. 


56 


ACCESSOKY  APPARATUS. 


by  the  number  of  turns  and  parts  of  a turn  given  to  the  screw  to 
cause  the  image  of  the  object  to  pass  entirely  across  the  cobweb. 

For  using  this  micrometer,  the  stage  (if  movable)  is  made 
fast  by  a clamp.  The  object-slide  is  placed  upon  the  plate  A 
A,  which  is  rotated  till  the  object  is  in  a proper  position  to  be 
measured.  One  edge  of  the  object  is  brought  to  coincide 
exactly  with  the  cobweb  in  the  field  of  view,  and  the  reading 
of  the  scale  v,  graduated  head  y,  and  vernier  n,  are  accurately 
noted,  or  if  the  object  is  a small  one,  the  graduated  head  y can 
at  once  be  set  to  zero  of  its  scale.  The  milled  head  is  then 
turned  till  the  object  has  passed  entirely  across  the  line  in  the 
eye -piece,  and  the  readings  of  the  scale,  graduated  head,  and 
vernier  are  again  examined.  The  difference  between  the  first 
and  second  readings  is  the  exact  measurement  of  the  object. 

The  great  accuracy  of  this  instrument,  and  the  facility  with 
which  it  can  be  used  on  any  microscope,  no  preliminary  calcu- 
lations being  required,  will  commend  it  to  the  favor  of  all  who 
desire  the  most  perfect  micrometer  yet  invented. 

INSTRUMENTS  FOR  DRAWING  WITH  THE  MICROSCOPE. 

74.  Wollaston’s  Camera  Ciicida  is  the  instrument  com- 
monly used  by  artists  for  sketching  from  nature.  Tliis  instru- 
ment is  fitted  to  the  eye-piece  of  the  microscope,  and  enables  the 
observer  to  sketch  upon  paper,  placed  on  the  table,  the  magni- 
fied image  of  any  object  seen  in  the  microscope.  It  consists  of 
a four  sided  prism  of  glass,  having  its  faces  and  angles  so 
arranged  that  light  entering  the  first  face  of  the  prism  is  totally 
refiected  by  the  second  and  third  faces,  and  emerges  perpen- 
dicular to  the  fourth  face  of  the  prism,  and  at  right  angles  with 
its  original  direction. 

When  using  this  camera^  the  microscope  is  placed  in  a hori- 
zontal position,  and  the  object  appears  projected  upon  the 
paper  placed  on  the  table  to  receive  it,  and  it  may  be  traced  by 
a pencil  which  the  eye  sees  at  the  same  time  as  the  object. 

75.  Wacliet’s  Camera  I^tieida.  This  instrument  consists 
of  a triangular  prism,  having  its  three  faces  and  angles  equal. 


J.  & W.  GRUNOW  & GO’S  ILLUSTRATED 


CAMERA  LUCIDA. 


57 


Fig.  25  sliow8  this  camera,  mounted  in  rig.  25. 

a cap,  which  fits  the  top  of  the  eye-piece. 

If  the  microscope  is  inclined  at  an  angle 
of  thirty  degrees  with  the  horizon,  this 
camera  may  be  used  in  the  Same  manner 
as  Wollaston^ s Camera  Lucida^  described 
in  the  previous  section. 

With  this  camera  the  microscope  is 
sometimes  placed  in  a . perpendicular  posittion,  and  the  drawing 
made  on  a table  which  is  inclined. 

76.  Soemmeriiig’§  Steel  Speculum.  This 
is  a plane  speculum  of  polished  steel,  smaller 
than  the  ordinary  pupil  of  the  eye,  commonly 
set  at  an  angle  of  45°,  and  mounted  as  shown 
in  Fig.  26,  and  attached  to  the  eye-piece  in  the 
same  manner  as  the  camera. 

This  speculum  is  so  mounted  that  its  angle  of  inclination 
may  be  changed  to  project  the  image  on  any  part  of  the  paper 
where  it  is  most  convenient.  With  this  instrument,  the 
reflected  image  of  the  object,  and  the  pencil,  both  appear 
together  on  the  paper,  and  the  microscope  may  be  placed  at 
any  angle  which  is  found  most  convenient. 

77.  Using  the  Camera.  In  using  the  camera  lucida,  or 
speculum,  some  care  is  required  to  see  both  the  object  and  the 
pencil  at  the  same  time.  Sometimes  the  light  on  the  paper  is 
to6  strong,  and  it  requires  to  be  shaded  by  a book  or  other 
object.  In  the  evening,  on  the  other  hand,  the  pencil  and 
paper  require  additional  illumination.  If  the  object  and  pencil 
are  not  both  visible  on  the  paper  at  the  same  time,  the  diffi- 
culty will  generally  be  overcome  by  moving  the  eye  about  till 
the  proper  position  is  found,  and  -when  once  obtained  the  eye 
should  be  kept  steadily  in  that  position  till  the  drawing  is  com- 
pleted. 

To  have  different  drawings  on  a uniform  scale,  it  is  neces- 
sary to  have  the  microscope  always  inclined  at  the  same  angle, 
that  is,  to  have  the  camera  at  a uniform  distance  above  the 
paper.  Nine  inches  will  generally  be  found  the  proper  dis- 


Fig.  26. 


CATALOGUE  OF  ACHROMATIC  MICROSCOPES. 


58 


ACCESSORY  APPARATUS. 


tance  for  easy  and  accurate  drawing.  The  size  of  the  picture 
will  vary  with  the  distance  of  the  paper  from  the  instrument. 

Experience  is  of  course  required  to  give  steadiness  to  the 
hand  and  to  secure  accurate  drawings,  but  it  is  often  so  desira- 
ble to  make  accurate  drawings  of  objects  seen  with  the  micro- 
scope, that  every  one  ought  to  practice  till  he  can  draw  with 
facility.  It  makes  little  difference  which  of  the  three  instru- 
ments described  is  used,  a person  draws  best  with  the  instru- 
ment to  which  he  has  become  accustomed. 

78.  Camera  I^acida  applied  to  Micrometry.  If  the 
microscope  is  placed  at  an  angle  convenient  for  using  the 
camera,  and  care  is  taken  to  adjust  it  always  in  the  same  posi- 
tion, the  lines  on  a stage  micrometer  may  be  projected  on 
paper,  by  means  of  the  camera,  and  form  an  accurate  scale 
for  measuring  any  object  drawn  with  the  same  power,  and 
with  the  instrument  in  the  same  position.  Supposing  the 
divisions  of  a stage  micrometer,  whose  real  values  are  2J0 
an  inch,  are  projected  with  such  a magnifying  power  as  to 
be  at  the  distance  of  one  inch  from  each  other  on  the  paper, 
it  is  obvious  that  if  any  object  is  delineated  on  the  paper  with 
the  same  power,  every  inch  of  the  drawing  corresponds  to 
2^5  of  an  inch  on  the  object,  J of  an  inch  would  equal  one 
thousandth  of  an  inch  in  the  object,  and  so  on.  We  may  there- 
fore draw  parallel  lines  on  the  paper,  subdividing  the  spaces 
formed  by  projecting  the  micrometer  lines,  and  the  scale  thus 
formed  will  serve  for  measuring  any  object  we  may  examine. 

A similar  scale  may  be  prepared  with  each  object-glass,  and 
by  viewing  any  object  through  the  camera,  with  the  scale 
placed  below,  we  determine  at  once  its  magnitude.  When 
sufficient  magnifying  power  is  used,  the  instrument  properly 
adjusted,  and  the  scale  thus  made  is  minutely  divided,  great 
accuracy  may  be  obtained. 

79.  Movable  Hiaphrag^m-Platc.  “ ISTo  microscope  stage 
(says  Dr.  Carpenter)  should  ever  be  without  a diaphragm- 
plate,  fitted  to  its  under  surface,  for  the  sake  of  restricting  the 
amount  of  light  reflected  from  the  mirror,  and  of  limiting  the 
angle  at  which  its  rays  impinge  on  the  object.”  This  appa- 
ratus, shown  at  Fig.  27,  is  attached  to  the  under  side  of  the 


J.  & W.  GRUNOW  & GO’S  ILLUSTRATED 


bull’s-eye  condenser. 


59 


stage  by  a bayonet  joint.  The  dia- 
phragm having  a variety  of  open- 
ings of  different  form  and  size,  turns 
upon  a pivot  so  situated  that  each  , 
opening  is  successively  brought  into  I 
the  axis  of  the  microscope.  The® 
space  between  the  largest  and  small- 
est openings  is  greater  than  between 
any  other  two,  and  is  designed  to  exclude  all  transmitted  light 
and  give  a dark  background  when  viewing  opaque  objects. 

A bent  spring  is  attached  to  the  fixed  part,  and  rubs  against 
the  edge  of  the  movable  plate,  which  is  provided  with  notches, 
so  arranged  that  when  either  of  the  holes  is  brought  into  its 
proper  position  the  end  of  the  spring  drops  into  the  notch. 

In  the  plain  form  of  diaphragm  all  the  openings  are  circular? 
but  in  the  more  expensive  kinds,  openings  of  a variety  of 
forms  are  employed,  some  excluding  the  central  rays,  others  are 
crescent  shaped,  or  semi-circular,  admitting  only  light  from  one 
side. 

When  the  eye  becomes  fatigued  by  too  strong  light,  or  by 
the  intense  glare  or  yellow  rays  of  artificial  light,  relief  is 
afforded  by  inserting  in  the  body  of  the  diaphragm  a piece  of 
gray,  neutral  tint,  or  light  blue  glass,  by  which  the  light  can 
be  modified  to  any  extent  required. 

80.  Bull’§-Eye  Condenser.  This  is  a large  plano-convex 
lens,  mounted  on  a brass  stand  and  pillar.  The  lens  is  attached 
by  a cradle  joint  to  a revolving  arm,  which  is  supported  by  a 
sliding  tube,  so  cut  as  to  act  as  a spring  and  retain  the  arm  and 
lens  steady  at  any  elevation  and  in  any  position.  For  illumina- 
ting opaque  objects,  the  bull’s-eye  is  so  adjusted  above  the 
stage,  with  its  fiat  side  towards  the  object,  as  to  bring  the  light 
to  a focus  upon  the  object  on  the  stage.  In  using  artificial  light, 
the  large  bull’s-eye  is  to  be  placed  with  its  plane  surface 
towards  the  light,  and  so  adjusted  that  the  beam  of  light 
transmitted  shall  be  about  the  size  of  the  mirror,  or  of  the 
smaller  bull’s-eye,  when  the  rays  will  be  nearly  parallel,  or 
slightly  converging.  The  mirror,  or  smaller  bull’s-eye,  may 
then  be  used  to  bring  the  light  to  a focus  upon  the  obj  ect.  If  the 


CATALOGUE  OF  ACHROMATIC  MICROSCOPES. 


60 


ACCESSOEY  APPARATUS. 


smaller  bull’s-eye  is  used 
to  bring  the  light  to 
a focus,  its  flat  surface 
should  be  turned  towards 
the  object. 

By  following  these 
directions,  very  little 
spherical  aberration  is 
produced  by  the  form  of 
the  bull’s-eye,  and  the 
illumination  is  rendered 
more  efiective  than  by 
any  other  position  of  the 
lenses. 


81-  Smaller  Biill’s-Eye 
Condenser.  This  instru- 
ment is  mounted  as  shown  in 
Fig.  29.  It  is  very  conven- 
ient for  illuminating  opaque 
objects  to  be  viewed  with 
low  powers.  The  method  of 
using  it  will  be  understood 
from  the  preceding  section. 


J.  & W.  GRUNOW  & CO’S  ILLUSTRATED 


ACHROMATIC  CONDENSER. 


61 


82.  Aclironiatic  Condenser.  For  low  powers  the  concave 
mirror,  placed  below  the  stage,  furnishes  all  the  light  which  is 
required,  but  for  developing  the  best  effect  of  the  higher  pow- 
ers, as  the  J,  J and  j2  i^^^h  objectives,  it  is  sometimes  necessary 
to  have  the  object  illuminated  with  achromatic  light  highly 
concentrated.  If  the  object-glass  has  an  angular  aperture  of 
from  ninety  to  one  hundred  and  sixty  degrees,  and  the  pencil 
of  light  by  which  the  object  is  illuminated  is  condensed  at  an 
angle  of  only  fifty  degrees,  (and  especially  if  it  is  also  affected 
with  spherical  and  chromatic  aberration,)  it  is  evident  that 
unless  the  object  itself  disperses  the  light,  there  will  be  no 
light  to  be  taken  up  by  the  marginal  part  of  the  object-glass. 
It  is  found  that  generally  objects  do  thus  disperse  the  light  to  a 
limited  extent,  but  that  to  secure  the  fullest  advantage  from 
object-glasses  of  large  angular  aperture,  it  is  necessary  to  illumi- 
nate the  object  with  a pencil  of  achromatic  light,  condensed  at 
an  angle  bearing  a considerable  proportion  to  the  aperture  of 
the  object-glass. 

This  object  is  secured  by  passing  the  illuminating  pencil 
through  an  achromatic  combination  of  lenses,  of  large  aper- 
ture, placed  beneath  the  stage. 


Fig.  30. 


The  achromatic  condenser  is  so  mounted  that  its  focus  can  be 
easily  adjusted  to  the  exact  position  of  the  object.  The  appa- 
ratus in  which  it  is  mounted,  is  shown  in  Fig.  30,  and  is 
attached  to  the  under  side  of  the  stage  by  a bayonet  catch 
shown  in  the  upper  part  of  the  figure.  An  inner  cylinder, 


CATALOGUE  OF  ACHROMATIC  MICROSCOPES. 


62 


ACCESSORY  APPARATUS. 


which  carries  the  condenser,  is  moved  by  rack  and  pinion  by 
turning  the  milled  head,  which  serves  to  adjust  the  focus. 

The  achromatic  condenser  fits  to  the  inner  tube  by  a bayonet 
joint,  and  when  it  is  removed,  a diaphragm,  or  N'achefs 
Oblique  Prism^  may  be  inserted  in  the  same  brass  work. 
The  achromatic  condenser  furnishes  a pencil  of  light  which  is, 
1st,  free  from  color ; 2nd,  free  from  spherical  aberration  ; 3rd, 
condensed  at  a very  large  angle.  These  qualities  render  it 
exceedingly  valuable  for  displaying  delicate  objects  viewed 
with  the  higher  powers. 

To  use  this  condenser,  first  place  an  object  on  the  stage, 
illuminating  it  by  means  of  the  concave  mirror,  and  with  a low 
power  bring  the  object  into  exact  focus;  now  remove  the 
object  without  altering  the  focus  of  the  object-glass,  attach  the 
achromatic  condenser  and  illuminate  it  by  means  of  the  plane 
mirror;  now  by  turning  the  milled  head,  so  adjust  the  con- 
denser that  the  image  of  window  bars  or  trees  may  be  seen  in 
the  microscope ; then  turn  the  mirror  so  as  to  reflect  light  from 
a white  cloud,  or  other  strong  light,  through  the  condenser ; 
then  place  the  object  on  the  stage  and  attach  one  of  the  higher 
powers  to  the  microscope,  and  when  the  object  is  brought  into 
focus  it  will  be  seen  most  beautifully  illuminated,  and  delicate 
structures,  scarcely  distinguishable  by  other  methods  of  illumi- 
nation, will  be  clearly  defined. 

83.  ]Vactiet’§  Prii^m  for  OMique  Illumiiiatiou.  This 
prism  is  so  contrived  as  to  illuminate  the  object  with  a pencil 
of  light  from  one  side  of  the  axis,  the  form  of  the  prism  being 
previously  constructed  so  as  to  give  to  the  light  any  angle  of 
inclination  required.  Let  M O,  Fig.  31,  be  the  axis  of  the 
microscope,  h c d q are  the  four  sides  of  ITachet’s 
prism,  'M?  O is  the  angle  at  which  the  pencil  of  light  is  re- 
quired to  fall  upon  the  object  O.  To  the  side  d q^  is  cemented 
a lens,  having  such  a focus  as  to  condense  the  light,  transmitted 
by  the  prism,  upon  the  object  O,  on  the  stage  of  the  micro- 
scope. This  prism  is  set  in  a piece  of  tube  C C,  a diaphragm 
placed  below  cuts  off  all  the  light  whicli  would  not  pass 
through  the  prism  and  be  brought  to  a focus  at  O. 

When  this  prism  is  used  a beam  of  light  L,  is  reflected 


J.  & W.  GRUNOW  & GO’S  ILLUSTRATED 


LIBERKUHN  SPECULUM. 


63 


upwards  by  the  plane  mirror  M,  so  as  to  enter  the  prism.  It 
then  suffers  total  reflection  in 
the  direction  w and  is  again 
reflected  by  the  opposite  force 
of  the  prism  in  the  direction  v 
O ; as  it  emerges  from  the  con- 
vex surface  of  the  lens  c?  it 
is  condensed  and  brought  to  a 
focus  upon  the  object  O,  which 
is  in  the  focus  of  the  object- 
glass  immediately  above  it. 

This  prism  is  so  mounted  be- 
low the  stage  of  the  micro- 
scope that  it  can  be  revolved 
to  give  light  from  any  direc- 
tion, or  illuminate  successive- 
ly every  side  of  an  object. 

This  method  of  illumination 
brings  out  many  delicate 
markings,  and  reveals  pecu- 
liarities of  structure  not  otherwise  appreciable.  See  Section  16. 

When  the  microscope  is  furnished  with  an  achromatic  con- 
denser, bTachet’s  prism  can  be  inserted  in  the  same  brass  work. 

84.  liieberkubn  Speculum.  This  instrument,  shown  in 
Fig.  32,  consists  of  a small  concave  metallic  reflector  L L, 
attached  to  a short  tube  which  is  fitted  to  the  lower  part  of  the 
object-glass  A.  The  polished  surface  presents  that  degree  of 
concavity  which  is  adapted  to  bring  to  a focus,  upon  the  object, 
the  beam  of  parallel  light  reflected  from  the  plane  mirror 
below  the  stage.  These  little  reflectors  are  called  Lieberkuhns, 
from  the  name  of  their  inventor. 

The  rays  of  light  reflected  from  the  mirror  B,  Fig.  32,  pass 
through  that  part  of  the  slide  S S,  not  covered  by  the  object, 
and  being  again  reflected  by  the  Lieberkuhn  L L,  are  brought 
to  a focus  upon  the  object,  which  is  then  seen  by  reflected  light. 

If  the  object  is  transparent,  the  small  stop  or  darh  well  D, 
gives  a black  ground  behind  the  object,  which  is  then  seen 


CATALOGUE  OF  ACHROMATIC  MICROSCOPES. 


64: 


ACCESSORY  APPARATUS. 


brilliantly  illuminated  upon  a 
dark  field.  The  dark  well  is 
made  in  the  form  of  a cup,  in 
order  that  the  bottom  may 
not  be  sufficiently  illuminated 
to  form  a light  ground  to  the 
object,  which  might  happen 
if  the  disk  were  employed. 

Three  sizes  of  these  dark 
wells  are  usually  supplied,  the 
largest  being  always  used 
with  the  lower  powers. 

The  Lieberkuhn  gives  a 
full  and  uniform  illumination 
on  every  side  of  the  object, 
while  the  bull’s-eye  gives 
only  oblique  light  from  one 
direction.  The  Lieberkuhn 
is  also  available  to  illuminate 
opaque  objects  viewed  with  object-glasses  of  such  short  focus 
as  to  preclude  the  use  of  the  bull’s-eye. 

85.  The  Erector  is  inserted  in  the  lower  end  of  the  draw- 
tube  of  the  microscope.  Its  use  is  to  reverse  the  position  of 
the  image,  (which  is  inverted  in  the  compound  microscope.)  so 
that  it  shall  appear  in  the  true  position  of  the  object.  It  is 
used  with  low  powers  for  dissecting  and  other  manipulations, 
where  the  hands  require  to  be  guided  by  looking  through  the 
microscope. 

86.  Among  the  recent  valuable  additions  to  the  microscope, 
should  be  mentioned  the  Ortlioscopic  Eye-piece,  invented  by 
Mr.  Charles  Kellner,  optician  of  Wetzlar.  It  is  adapted  to  micro- 
scopes, and  also  to  telescopes  of  all  kinds,  the  dialy  tic  included. 
It  gives  a large  field  of  view,  free  from  curvature  or  distortion 
of  any  kind,  perspectively  correct,  with  sharpness  of  definition 
throughout  its  whole  extent,  without  the  blue  ring  which  en- 
circles the  borders  of  the  field  in  the  ordinary  negative  eye-piece. 


Fig.  32. 


J.  & W.  GRUNOW  & GO’S  ILLUSTRATED 


ANIMALCULE  CAGE  WITH  SCKEW. 


65 


87.  Compressor.  It  is  often  required  to  tear  up  delicate 
portions  of  tissue  upon  the  field  of  the  microscope,  or  to  fioat 
them  out,  as  it  were,  from  the  general  substance  under  exam- 
ination, by  pressing  down  the  thin  glass  which  covers  them  ; 
many  parts  of  plants  are  also  better  seen  when  slightly  com- 
pressed. 

Fig.  33. 


The  compressor,  Fig.  33,  enables  us  to  apply  any  amount  of 
graduated  pressure  upon  the  thin  glass  which  covers  the  object. 
The  lever  bears  at  one  end  a fiat  brass  ring,  which  moves  on  a 
universal  joint,  and  which  can  be  elevated  or  depressed  by 
turning  the  screw  at  the  other  end  of  the  lever.  When  the 
screw  is  loosened  the  lever  can  be  turned  around  on  the  pivot 
which  secures  it  to  the  plate ; the  ring  being  thus  turned  away, 
the  object  on  the  glass  plate,  which  covers  the  opening  in  the 
compressor,  can  be  changed. 

If  desired,  an  ordinary  slide  can  be  placed  upon  the  plate  of 
the  compressor,  and  the  cover  pressed  down  upon  the  obj  ect  on 
the  slide.  Generally  a plate  of  glass  is  cemented  over  the 
opening  in  the  compressor,  and  a circle  of  thin  glass  is  cemented 
to  the  movable  ring  attached  to  the  lever.  These  glasses  can 
be  easily  removed,  if  desired,  or  their  places  supplied  by  new 
ones,  if  they  chance  to  be  broken. 

88.  Atiimalciile  Cage  witli  Screw.  The  animalcule  cage 
shown  at  Fig.  31,  consists  of  ^4. 

a brass  plate,  or  slide,  carry- 
ing a short  cylinder,  which 
supports  a circular  plate  of 
glass,  over  which  fits  a cap 
bearing  a circle  of  thin  glass  / a screw  collar  retains  the  cap  in 
place,  and,  when  screwed  down  upon  it,  produces  moderate 

CATALOGUE  OF  ACHROMATIC  MICROSCOPES. 

5 


66 


-ACCESSOEY  APPAEATUS. 


pressure  sufficient  to  retain  any  animalcules  in  place  while  they 
are  examined. 

This  instrument  may  also  be  used  in  some  cases  instead  of 
the  “ compressor. 

89:  Tlie  Simple  Animalcule  Cage  differs  from  the  prece- 
ding only  in  having  the  cap  retained  in  place  by  a cylindrical 
ring,  so  cut  as  to  act  as  a spring.  The  use  of  this  instrument  is 
the  same  as  the  preceding,  though  the  amount  of  pressure 
obtained  by  it  is  somewhat  less. 

90.  Stage  Forceps.  The  very  convenient  instrument  shown 
in  Fig.  35,  can  be  attached  to  the  stage  and  made  fast  by  turn- 
ing the  screw  with  a milled  head. 

These  forceps  slide  like  a pencil  in  a cylindrical  support, 
while  the  jointed  arm  and  pivot  allow  of  motion  in  any 

Fig.  35. 


direction.  Minute  insects  or  other  objects  can  be  held  by  this 
instrument  in  any  position  required.  One  end  of  the  instru- 
ment carries  a needle  which  can  be  used  for  the  same  i^urpose. 

Hand  Forceps,  both  of  brass  and  of  steel,  to  be  used  with 
the  microscope,  are  furnished  to  order. 

91.  Frog  Plate.  For  viewing  the  circulation  of  the  blood, 
the  most  convenient  subject  is  the  common  frog.  The  capillary 
circulation  in  the  thin  transparent  web  of  the  foot,  or  in  the 
tongue,  affords  the  most  interesting  exhibition  the  microscopist 
can  enjoy.  Some  care  is  required  in  so  arranging  the  little  an- 
imal as  to  avoid  giving  him  pain,  or  even  stopping  the  circula- 
tion., by  undue  pressure  on  some  part  of  the  circulatory  system. 
The  Feog  Plate  is  an  instrument  devised  to  secure  these  ob- 
jects. An  extra  stage  or  brass  plate,  about  two  and  a half  by 
three  inches,  having  a central  opening,  is  attached  to  the  ordi- 
nary stage  of  the  microscope  by  a short  piece  of  tube  attached 


J.  & W.  GRUNOW  k GO’S  ILLUSTRATED 


MACHINE  FOR  CU'ITING  CIRCLES  OF  THIN  GLASS. 


67 


to  its  centre,  and  so  cut  as  to  act  as  a spring  when  inserted  in 
the  opening  of  the  stage.  An  axis  extends  horizontally  from 
one  side  of  the  brass  plate  above  described,  to  which  is  attach- 
ed a plate  of  wood  about  three  inches  square,  on  which  the 
frog  is  secured  in  a bag.  This  plate  carrying  the  frog,  can  be 
rotated  so  as  to  give  to  the  animal  the  most  easy  position  for 
extending  the  foot,  so  as  to  allow  the  web  to  be  spread  out  upon 
a glass  plate  over  the  opening  in  the  stage  of  the  microscope. 
Oil  the  opposite  side  of  the  brass  plate  are  sliding  sockets,  in 
which  are  inserted  pivots  or  keys  like  those  which  tighten  the 
strings  of  a violin.  To  these  pivots  or  keys  are  attached  the 
threads  which  extend  the  toes  of  the  frog.  The  threads  may 
be  tightened  by  turning  the  keys,  or  their  position  changed  by 
moving  the  sliding  sockets.  This  instrument  enables  one  to 
view  the  circulation  in  the  frog  with  great  facility. 

92.  ITlacliiiie  for  Cutting  Circles  of  Tliiii  Glass.  The 


base  of  this  instrument,  which  is  of  mahogany,  supports  a strong 
bent  arm  of  japanned  cast  iron.  From  the  end  of  this  arm, 
firmly  attached  to  it,  projects  downward  a cylindrical  guide, 
two  inches  long,  designed  to  steady  the  other  parts  of  the  ap- 


CATALOGUE  OF  ACHROMATIC  MICROSCOPES. 


68 


ACCESSORY  APPARATUS. 


paratus.  Through  this  cylinder  passes  a steel  stem,  ending  be- 
low ill  a conical  base  of  brass,  in  the  bottom  of  which  is  insert- 
ed a piece  of  cork  or  india  rubber  to  press  upon  the  thin 
glass  and  hold  it  steady.  At  the  top  is  a milled  head  «,  under 
which  is  a helical  spring,  which  supports  the  stem,  and  lifts  it 
from  the  glass  when  not  in  use.  Outside  the  cylindrical  guide 
is  a tube  o,  carrying  a milled  head  and  the  socket  n,  to 
which  is  attached  the  bent  arm  c,  carrying  the  waiting  dia- 
mond d.  The  diamond  is  secured  by  a screw,  and  the  arm 
which  carries  it  can  be  extended  at  pleasure,  and  secured  by 
the  screw  attached  to  the  socket.  The  brass  cone  at  the  base 
of  the  stem  supports  the  tube  c>,  but  the  tube  can  be  raised  on 
the  cylindrical  guide,  so  that  the  diamond  presses  upon  the 
glass  only  by  the  weight  of  the  arm  <?,  and  tube  o. 

To  use  this  instrument,  set  the  diamond  at  such  a distance 
from  the  axis  as  is  required  to  make  the  circular  covers  of  thin 
glass  of  suitable  diameter,  lay  the  thin  glass  to  be  cut  upon  a 
plate  of  glass  previously  moistened  so  as  to  make  the  thin  glass 
adhere  ; slide  the  plate  carrying  the  thin  glass  under  the  dia- 
mond, and  hold  it  firmly  by  placing  a finger  upon  the  milled 
head  a ; the  diamond  will  then  rest  gently  upon  the  glass,  and 
may  be  revolved  by  turning  the  milled  head  h.  Kemoving 
the  pressure  from  move  the  plate  and  thin  glass  to  a position 
convenient  for  cutting  another  circle,  and  so  continue  till  the 
piece  of  thin  glass  has  been  all  cut  into  circles.  When  the 
plate  is  removed  the  circles  of  thin  glass  can  be  easily  separa- 
ted. The  thin  glass  lies  more  firmly  in  its  place,  and  is  less 
liable  to  crack  when  laid  upon  a plate  of  wet  glass,  than  if  laid 
upon  wood  or  dry  glass.  In  turning  the  milled  head  5,  care 
should  be  taken  not  to  press  too  heavily  upon  the  diamond 
point.  A very  light  cut  only  is  required,  while  a heavy  scratch 
is  apt  to  fracture  the  glass. 

Glass  covers  may  be  cut  of  different  sizes,  so  as  to  use  up  all 
irregular  fragments  of  thin  glass,  which  could  not  be  profitably 
cut  into  squares. 

Circular  covers  in  general  look  better  upon  microscopic  ob- 
jects than  squares,  and  with  this  instrument  they  may  be  cut 


J.  k W.  GRUNOW  & GO’S  ILLUSTRATED 


INSTRUMENT  FOR  MAKING  CELLS  OF  GOLD  SIZE. 


69 


with  great  facility  and  economy,  as  more  circles  can  be  cut 
from  the  same  glass,  than  squares  of  equal  breadth. 

93.  Instrument  for  makings  Cells  of  gold  size,  or  of 
other  fluids.  In  mounting  microscopic  objects  very  shallow 
cells  are  often  required,  which  are  most  conveniently  made  of 
some  fluid  cement,  as  gold  size,  japan  varnish,  or  some  other 
similar  material.  This  is  effected  by  placing  the  slide  upon  a 
revolving  table,  and  with  a suitable  brush  laying  on  the  cement 
evenly  in  a circle  in  the  centre  of  the  slide,  as  it  revolves  upon 
the  table. 

The  instrument  used  for  this  purpose  consists  of  a small  slab 
of  mahogany,  into  one  end  of  which  is  fixed  a pivot,  whereon  a 
circular  turn-table  of  brass,  about  three  inches  in  diameter,  is 
made  to  rotate  easily,  a rapid  motion  being  given  to  it  by 
applying  the  fore-finger  to  a milled  head  below  the  revolving 
table.  A circle  about  an  inch  in  diameter,  traced  upon  the 
centre  of  the  table,  serves  to  centre  the  glass  slide  which  is 
laid  upon  it,  and  spring-clips  retain  the  slide  on  the  table.  A 
camel-hair  pencil,  dipped  in  the  varnish  to  be  used,  is  held  in 
the  right  hand  and  its  point  applied  to  the  slide  at  a proper 
distance  from  the  centre ; the  table  is  then  rapidly  rotated  with 
the  left  hand  and  a ring  of  varnish  of  suitable  breadth  is  easily 
made  upon  the  glass.  The  slide  is  then  set  away  to  dry,  when, 
if  a thicker  cell  is  required,  another  coat  of  varnish  may  be 
applied  in  the  same  manner. 


CATALOGUE  OF  ACHROMATIC  MICROSCOPES. 


70  POLARIZED  LIGHT  AND  ITS  APPLICATION  TO  THE  MICROSCOPE. 


CHAPTER  IV. 


POLARIZED  LIGHT  AND  ITS  APPLICATION  TO  THE  MICRO- 
SCOPE. 

9-1.  Tlicories  of  Light.  Formerly  light  was  supposed  to 
consist  of  material  particles,  or  corpuscles,  projected  in  all 
directions  from  luminous  bodies.  This  theory  is  generally 
ascribed  to  Sir  Isaac  I7ewton,  but  I7ewton  really  held  that  in 
addition  to  the  projection  of  luminous  corpuscles,  each  moving  I 
particle  in  its  flight  produced  vibrations  in  the  surroundins 
ether,  similar  to  the  waves  produced  by  a stone  fallingdnto  the/ 
w’ater.  I 

Huyghens  maintained,  in  opposition  to  Newton,  that  lighf 
consisted  solely  in  vibrations  of  an  etherial  medium,  originated 
by  luminous  bodies,  without  the  onward  progress  of  any  sub- 
stance whatever.  With  some  modifications,  the  theory  of 
Huyghens  is  generally  adopted  by  men  of  science  at  the 
present  time,  though  the  honor  of  developing,  and,  to  a great 
extent,  demonstrating,  the  undulatory  theory  of  lights  is  due  to 
Dr.  Young,  Sir  David  Brewster,  and  other  modern  philosophers. 

95.  Double  Refraction  and  Polarized  Light  have  con- 
stituted the  great  battle  ground  of  science  in  advancing  th^ 
claims  of  rival  theories  of  light.  Some  principal  facts  in  r^ 
gard  to  the  polarization  of  light  by  the  double  refraction  df 
Iceland  spar,  were  known  to  Newton  and  Huyghens,  but  tpe 
development  of  the  Science  of  Polarized  Lights  dates  from  me 
early  part  of  the  present  century. 

In  1808,  the  Royal  Institute  of  France  offered  a prize  for/the 


J.  & W.  GRUNOW  k GO’S  ILLUSTRATED 


POLARIZATION  BY  REFLECTION. 


71 


best  memoir  giving  the  mathematical  theory  of  double  refrac- 
tion in  various  crystals,  demonstrated  by  experiment.  This 
prize  was  awarded  in  1810,  to  M.  Malus,  Lieutenant  Colonel 
in  the  Imperial  Guards,  and  member  of  the  Egyptian  Institute. 
In  1808,  while  Malus  was  engaged  in  his  experiments  on 
double  refraction,  and  was  casually  observing  through  a double 
refracting  prism  the  light  of  the  setting  sun  as  it  came  reflected 
at  a certain  angle  from  the  windows  of  the  Luxembourg 
Palace,  in  Paris,  on  slowly  revolving  the  prism  between  his 
eye  and  the  light,  he  was  surprised  to  see  a remarkable  differ- 
ence in  the  brilliancy  of  the  two  images,  the  most  refracted 
gradually  changing  from  brightness  to  obscurity,  and  the  re- 
verse at  each  quadrant  of  revolution.  A phenomenon  so  un- 
expected led  him  to  investigate  its  cause,  and  in  the  progress  of 
his  inquiries  he  discovered  that  every  substance  in  nature, 
having  a polished  surface,  is  capable  of  polarizing  light  by  re- 
flection at  a specific  angle  peculiar  to  each  substance.  The 
impetus  thus  given  to  scientific  curiosity,  attracted  to  the  study 
of  polarized  light  a constellation  of  talent  almost  unrivaled  at 
any  period  in  the  history  of  science.  So  extensive  have  been 
the  investigations,  and  so  wonderful  the  phenomena  discovered, 
that  the  subject  of  polarized  light  now  justly  ranks  as  one  of 
the  most  elegant  and  refined  branches  of  physical  optics. 

The  phenomena  of  polarized  light  will  here  be  described 
only  so  far  as  to  enable  the  reader  to  understand  their  applica- 
tion to  investigations  connected  with  the  microscope.  Brews- 
ter’s Optics,  Pereira’s  Lectures,  and  other  elaborate  treatises  in 
common  use,  will  furnish  fuller  details  to  those  who  desire  to 
pursue  the  subject. 

96.  Polarization  toy  reflection.  When  a ray  of  light  is 
reflected  from  a plate  of  glass,  or  the  polished  surface  of  any 
other  substance,  it  can  usually  be  reflected  again  from  another 
similar  surface,  and  it  will  pass  freely  through  transparent 
bodies.  If,  however,  a ray  of  light  be  reflected  from  a plate  of 
glass,  at  an  angle  of  56°  45',  it  becomes  incapable  of  reflection 
at  the  surface  of  another  plate  of  glass  in  certain  definite  posi- 
tions, but  it  will  be  completely  reflected  by  the  second  plate  in 


CATALOGUE  OF  ACHROMATIC  MICROSCOPES. 


72 


POLAKIZED  LIGHT  AND  ITS  APPLICATION  TO  THE  MICEOSCOPE. 


Other  positions.  It  also  loses  the  property  of  penetrating  trans- 
parent bodies  in  particular  positions.  Light,  so  modified,  is 
said  to  be  Polarized. 

Light  may  be  polarized  by  reflection  y'mTTi  any  polished  sttr- 
face.,  at  an  angle  peculiar  to  each  substance,  e.  g.,  from  glass  at 
56°  45',  from  water  at  53°  11',  rock  crystal  56°  58',  diamond 
68°  1'. 

From  a very  extensive  series  of  experiments,  made  to  deter- 
mine the  maximum  polarizing  angles  of  various  bodies.  Sir 
David  Brewster  arrived  at  the  following  law.  The  index  ofre- 
f raction  is  the  tangent  of  the  angle  of  polarization.  It  follows. 


therefore,  as  a geometrical  consequence,  that  the  reflected 


polarized  ray  forms  a right  angle  with  the  refracted  ray.  It 
is  generally  known  that  only  a part  of  the  light  falling  upon  a 
polished  surface  is  reflected  ; the  remainder  is  either  dispersed  or  / 

absorbed,  or,  if  the  body  is  transparent,  it  is  transmitted.  How  / 
as  the  differently  colored  rays,  of  which  ordinary  white  light  is 
composed,  are  not  refracted  equally  by  any  transparent  medium, 
it  follows  that  the  index  of  refraction,  and  consequently  the 
polarizing  angle,  varies  for  the  differently  colored  rays,  there- 
fore a ray  of  white  light  will  not  be  perfectly  polarized  by  re- 
flection at  any  angle,  but  there  is  a certain  range  of  angle 
within  which  the  polarization  is  more  or  less  perfect,  a portion 
of  light  remaining  unpolarized  even  at  the  angle  of  most  per- 
fect polarization. 

97.  Polarization  by  refraction.  Certain  crystallized  j 
minerals  and  salts  have  the  property  of  polarizing  light  trans- 
mitted through  them,  owing  probably  to  some  physical  pecu-  / 
liarity  in  the  form  or  crystalline  arrangement  of  their  ultimate 


molecules.  Such  are  tourmaline,  calc  spar,  (Iceland  spar,)  / 
quartz,  and  also  many  artificial  salts,  of  which  sulphate  of  j 
iodine  and  quinine  (Ilerapathite)  is  the  most  remarkable./ 
The  transparent  brownish  varieties  of  tourmaline  furnish  T ' 
most  convenient  polarizing  plates.  For  this  purpose  the  to 
maline  crystal  is  cut  into  plates  about  of  an  inch  in  thii 
ness  and  polished,  the  plane  of  section  being  parallel  to  t 
vertical  axis  of  the  hexagonal  prism,  in  which  form  this  ni 


J.  & W.  GRUNOW  & GO’S  ILLUSTRATED 


POLARIZATION  BY  REFRACTION. 


73 


era!  occurs.  A beam  of  common  light,  transmitted  through 
such  a plate,  is  almost  perfectly  polarized,  and  refuses  to  pass 
through  glass  and  other  transparent  media,  when  it  falls  upon 
them  in  certain  positions. 

The  beam  of  light  thus  polarized  will  pass  freely  through  a 
second  tourmaline  plate,  held  in  the  same  position  as  the  first, 
as  showm  at  Fig.  37. 

Fig.  SY.  Fig.  38. 


■k' 


But  if  one  of  the  plates  is  rotated  before  the  other,  and  in  a 
plane  parallel  with  it,  the  light  gradually  diminishes  till  the 
two  plates  are  at  right  angles  with  each  other,  as  in  Fig.  38, 
when  the  light  becomes  wholly  obscured.  As  the  rotation  con- 
tinues the  beam  gradually  reappears,  and  when  half  a revolu- 
tion has  been  performed  the  light  resumes  its  original  intensity. 

This  is  best  illustrated  by  looking  at  a candle  through  the 
tourmaline  plates.  If  one  plate  only  is  used,  the  candle  will 
be  distinctly  seen  in  every  position  of  the  tourmaline.  If  a 
second  plate  of  tourmaline  is  held  with  its  axis  parallel  to  the 
axis  of  the  first,  the  candle  will  still  be  seen  as  before,  but  if 
one  plate  is  slowly  rotated  before  the  other,  as  described,  the 
image  of  the  candle  will  slowly  vanish  and  reappear  alternately 
at  every  quarter  and  half  revolution  of  the  plate,  varying 
through  all  degrees  of  brightness  to  total,  or  almost  total, 
obscurity.  These  changes  depend  obviously  upon  the  relative 
position  of  the  plates,  and  upon  the  ultimate  form  or  physical 
properties  of  the  crystalline  particles  of  the  mineral. 

When  the  axes  of  the  two  plates  are  parallel,  the  brightness 
of  the  image  is  at  its  maximum,  and  when  the  axes  of  the  sec- 
tions cross  at  right  angles,  as  in  Fig.  38,  the  image  of  the 
candle  vanishes. 

A ray  of  light,  polarized  by  reflection,  when  examined  by  a 


CATALOGUE  OF  ACHROMATIC  MICROSCOPES. 


74  POLARIZED  LIGHT  AND  ITS  APPLICATION  TO  THE  MICROSCOPE. 


plate  of  tourmaline,  presents  the  same  phenomena  as  a ray 
originally  polarized  by  passing  through  a tourmaline  plate. 
From  its  great  convenience  a plate  of  tourmaline  is  commonly 
used  as  a test  of  polarized  light,  and  when  so  used  it  is  called 
an  analyzer. 

On  account  of  the  difficulty  of  obtaining  plates  of  tourma- 
line of  sufficient  size  and  freedom  from  color,  other  apparatus 
have  been  devised  for  the  same  purpose. 

98.  Polarization  by  refraction  tliroiigli  numerous 
plates  of  transparent  media.  If  light  is  transmitted  ob- 
liquely through  a bundle  of  thin  transparent  plates,  it  is  polar- 
ized. Plates  of  the  very  thin  glass  used  for  covering  micro- 
scropic  objects,  form  the  best  polarizer  of  this  kind.  Sixteen 
or  more  of  these  plates,  quite  clean,  are  to  be  placed  close 
together,  and  the  bundle  or  package  then  fixed  at  an  angle  of 
56°  45',  to  the  ray  to  be  polarized.  Common  window  glass,  or 
plates  of  mica,  may  be  used  in  the  same  manner,  but  the 
polarization  is  not  as  complete  as  that  produced  by  the  action 
of  the  thin  glass  above  referred  to. 

Light,  which  has  been  transmitted  through  such  a system  of 
thin  plates,  is  almost  entirely  polarized.'^*  It  may  be  reflected 
from  polished  surfaces  in  certain  positions,  but  not  in  others ; it 
may  also  be  transmitted  through  an  analyzer  in  one  position, 
but  it  is  wholly  intercepted  when  the  analyzer  is  rotated  90°. 

99.  Double  Refraetiou.  It  is  well  known  that  when  light 
passes  obliquely  through  w^ater  and  other  fluids,  glass  of  uni- 
form density,  common  salt  and  all  cubical  crystals,  or  crystals 
w’hose  form  is  derived  from  the  cube,  (called  monometric  crys- 
tals,) the  light  is  bent  out  of  its  course,  but  the  object  from 
which  the  light  proceeds  appears  single,  and  in  its  true  propor- 
tions if  the  transparent  medium  is  bounded  by  parallel  surfa- 
ces. Tliis  is  called  single  refraction.  Long  before  tlie  discov- 
ery of  polarized  light,  it  was  known  that  certain  bodies,  of 
which  the  most  noted  is  Iceland  spar,  give  two  images  of  all 
objects  seen  through  them  in  certain  directions.  It  is  now 

* The  light  which  remains  unpolarized  by  this  method,  can  only  be  appreciated 
by  the  most  delicate  analysis. 


J.  & W.  GRUNOW  & GO’S  ILLUSTRATED 


DOUBLE  REFKACTION  OF  ICELAND  SPAR. 


75 


known  that  all  crystals  whose  three  axes  are  not  at  right  angles 
with  each  other,  give  double  images  of  objects  seen  through 
them  in  some  directions.  This  property  is  called  double  re- 
fraction. 

A great  variety  of  substances  not  crystalline,  possess  the 
power  of  double  refraction, — animal  substances,  such  as  hairs, 
horns,  shells,  bones,  muscles,  nerves,  and  other  tissues  ; vegeta- 
ble substances,  like  certain  seeds,  starch,  gums,  resins,  essential 
oils  and  sugar  in  a fluid  state,  and  many  artificial  substances, 
as  glass  unequally  tempered.  In  many  of  these  substances  the 
separation  of  the  two  images  is  so  slight  that  the  double  refrac- 
tion is  not  ordinarily  perceived.  The  peculiar  character  of 
such  substances  will  be  better  understood  in  connection  with 
the  sections  on  Par'tial^  and  Colored  Polarization. 

The  actual  separation  of  the  two  images  produced  by  double 
refraction,  varies  greatly  in  different  substances ; to  show  the 
effect,  therefore,  a considerable  thickness  of  the  substance  is 
generally  required : transparent  crystals  afford  the  best  illus- 
trations of  this  peculiar  property. 

100.  Double  refraction  of  Iceland  Spar.  Iceland  spar, 
otherwise  known  as  crystallized  carbonate  of  lime^  calcareous 
spar,  or  calcite,  exhibits  in  a beautiful  manner  the  phenom- 
ena of  double  refraction.  It  crystallizes  in  the  form  of  oblique 
rhombic  prisms,  as  shown  in  Fig.  39.  39. 

It  is  bounded  by  six  equal  faces,  all 
rhombs,  meeting  each  other  at  angles 
of  105°  5',  or  its  supplement  71°  55'. 

It  has  three  axes,  one  of  which,  called 
the  vertical  or  major  axis,  a b,  Fig.  39, 
is  equally  inclined  to  each  of  its  six 
faces,  at  an  angle  of  45°  23'.  A plane 
a cb  d,  joining  two  obtuse  lateral  edges,  is  called  its  principal 
section.  This  crystal  cleaves  perfectly  parallel  to  either  of  its 
six  natural  faces. 

This  mineral  is  very  transparent,  and  in  pure  specimens 
quite  colorless  and  free  from  fractures.  Any  object,  as  a line 
or  point,  seen  through  any  of  the  faces  of  this  crystal,  in  any 


CATALOGUE  OF  ACHROMATIC  MICROSCOPES. 


76  POLARIZED  LIGHT  AND  ITS  APPLICATION  TO  THE  MICROSCOPE. 


direction  except  in  a line  parallel  to  a 5,  will  appear  double, 
owing  to  the  subdivision  of  the  pencil  into  two  beams.  As  the 
imaginary  line  or  axis  a is  that  in  which  no  doubling  of  the 
image  takes  place,  it  might  with  propriety  be  called  the  axis  of 
NO  double  refraction.^  but  as  the  amount  of  separation  of  the 
two  rays  depends  on  their  position  with  reference  to  a Z>,  that 
line  is  termed  the  axis  of  double  refraction.^  on  all  sides  of 
which  double  refraction  takes  place.  This  line  is  fixed  only  in 
direction  ; every  other  line  parallel  to  a J,  is  equally  a line  of  no 
double  refraction.  The  other  parts  of  the  crystal  can  be  split 
off  by  natural  cleavage  so  as  to  leave  any  line  parallel  to  a b, 
the  perpendicular  or  major  axis  of  the  remaining  crystal. 

Figure  40  shows  the  appearance  of  lines  A B,  C D,  E F, 
G H,  and  a circle  drawn  around  their  common  intersection, 
seen  through  a fiat  prism  of  Iceland  spar,  about  an  inch  and  a 
Fig.  40.  quarter  thick.  A B is 

parallel  to  the  principal 
section  of  the  prism  ex- 
pressed hj  a G b d,  Fig. 
39.  The  circle  and  lines 
are  all  seen  in  their  true 
position  when  viewed  in 
a direction  perpendicular 
to  the  face  of  the  prism, 
but  just  above  the  lines, 
and  parallel  to  them,  are 
seen  with  equal  bright- 
ness, the  dotted  lines  c d, 
efgh,  and  about  their 
common  intersection  a second  circle,  the  double  image  of 
the  first. 

The  pencil  of  light  is  divided  into  the  ordinary  and  extra- 
ordinary rays.  The  extraordinary  image  of  a line,  seen 
through  a double  image  prism,  is  always  parallel  to  the  ordi- 
nary image.  The  extraordinary  image  of  the  circle  shows 
clearly  that  every  point  is  displaced  in  a plane  parallel  to  A B, 
or  the  principal  section  joining  the  obtuse  lateral  edges  of  the 
prism.  Every  line  drawn  parallel  to  A B,  will  appear  single, 


J.  & W.  GRUNOW  & GO’S  ILLUSTRATED 


nicol’s  single  image  prism. 


77 


and  every  other  line  will  appear  double,  when  viewed  re- 
spectively in  a direction  perpendicular  to  a face  of  the  prism. 

101.  Polarization  produced  by  double  refraction.  If 
one  face  of  a prism  of  Iceland  spar  is  covered  by  an  opaque 
substance,  (as  paper  or  tin  foil,)  in  which  a small  hole  has  been 
pierced,  and  this  hole  is  viewed  from  the  opposite  face  of  the 
prism,  held  before  a beam  of  light,  two  illuminated  discs  will 
be  seen.  As  the  spar  is  revolved  before  the  eye,  one  image 
{fhe  ordinary  image)  remains  stationary,  while  the  other  {or 
extraordinary  image)  appears  to  revolve  around  the  first. 
Examined  by  an  analyzing  plate  of  tourmaline,  both  of  these 
images  are  found  to  be  perfectly  polarized.  Tliis  is  at  once 
evident  when  the  analyzer  is  rotated  in  front  of  the  stationary 
prism,  the  two  images  alternately  disappear  and  appear  again 
at  every  90°  of  the  revolution  of  the  tourmaline  plate,  one 
arriving  at  its  maximum  brightness  when  the  other  disap- 
pears, and  vice  versa,  the  maximum  brightness  of  both  images 
being  equal. 

If  now,  in  place  of  the  tourmaline,  a second  prism  of  Iceland 
spar  is  used  as  an  analyzer,  and  is  held  with  its  principal  sec- 
tion parallel  to  that  of  the  first  prism,  both  images  will  still  be 
seen,  but  separated  twice  as  far  as  when  one  prism  only  is  used. 
If,  now,  however,  the  second  prism  be  revolved  90®,  180°,  or 
270®,  only  one  image  remains.  But  in  all  other  positions  of 
the  second  prism,  each  of  the  images  produced  by  the  first 
prism  is  doubled,  so  that  four  images  will  be  seen  at  the  same 
time. 

The  ordinary  and  extraordinary  rays,  on  issuing  from  a doubly 
refracting  prism,  are  parallel  to  each  other,  and  it  is  clear  from 
the  preceding  observations,  that  they  are  polarized  in  planes  at 
right  angles  to  each  other.  A substance  having  such  proper- 
ties as  we  find  in  Iceland  spar,  must  be  of  great  value  as 
a means  of  analysis  and  polarization  of  light. 

102.  Nicol’s  Single  linage  Prism.  This  beautiful  con- 
trivance, devised  by  Mr.  Nicol  of  Edinburg,  is  of  the  greatest 
value  to  the  microscopist  of  all  known  means  of  polarization 
and  analysis,  since  it  furnishes  a perfectly  colorless  pencil  of 
polarized  light,  and  when  of  sufficient  size  allows  of  brilliant 


CATALOGUE  OF  ACHROMATIC  MICROSCOPES. 


78  POLARIZED  LIGHT  AND  ITS  APPLICATION  TO  THE  MICROSCOPE. 


illumination.  This  prism  is  constructed  from  an  elongated 
rhombohedron  of  Iceland  spar  in  the  following  manner : The 
natural  faceP  of  the  prism,  Fig.  41,  which  makes,  with  the  ob- 
tuse lateral  edge  K,  an  angle  of  about  71°,  is 
ground  away  so  as  to  form  a new  face, 
making  an  angle  of  68°  with  the  edge  K, 
and  a right  angle  with  the  plane  of  princi- 
pal section  joining  the  obtuse  lateral  edges 
K and  IP,  which  is  the  same  as  the  section 
a G I Fig.  39.  From  the  obtuse  solid 
angle  E,  the  prism  is  sawn  through  in  the 
direction  E F,  making  a right  angle  with 
the  new  terminal  face,  and  also  with  the 
plane  of  principal  section.  From  F another 
terminal  face  is  constructed  at  right  angles 
with  the  section  E F and  with  the  principal 
section,  and  making  an  angle  of  68°  with  the  edge  K'.*  All 
the  new  faces  are  now  carefully  polished,  and  the  two  parts  of 
the  prism  are  cemented  together,  in  their  former 
position,  with  Canada  balsam.  The  lateral  faces 
of  this  compound  prism  are  all  painted  black, 
leaving  only  the  terminal  faces  for  the  transmission 
of  light. 

Figure  42  represents  a section  of  Nicol's  prism 
through  the  obtuse  lateral  edges,  and  shows  the 
course  of  the  two  polarized  rays  into  which  com- 
mon light  is  divided  by  this  prism.  A ray  of  com- 
mon light,  a h,  entering  this  prism,  is  refracted 
into  the  ordinary  ray  h c,  and  the  extraordinary  ray 
h d.  The  index  of  refraction  of  Iceland  spar,  for 
the  ordinary  ray,  being  1.6543,  and  that  of  balsam 
only  1.536,  the  ordinary  ray  suffers  total  reflection 
at  the  surface  of  the  balsam,  and  cannot  pass  into 
the  lower  part  of  the  prism,  unless  the  incident 

* In  tlie  manufacture  of  Nicol’s  prisms,  the  inclination  of  the  terminal  faces  is 
sometimes  varied  to  suit  particular  purposes. 


Fig.  42. 


Fig.  41. 


J.  & W.  GRUNOAV  & CO’S  ILLUSTRATED 


THEORY  OF  POLARIZED  LIGHT. 


79 


ray  diverges  very  widely  from  the  axis  of  the  prism.  The 
extraordinary  ray  has  a refractive  power  so  low  that  it  is  not 
reflected  by  the  balsam,  unless  it  is  very  nearly  parallel  with 
its  surface,  but  passes  freely  through  it  into  the  lower  part  of 
the  prism  and  emerges  in  the  direction  g A,  parallel  to  the 
incident  ray.* 

Nicol’s  prisms  are  capable  of  transmitting  pencils  of  light, 
perfectly  polarized,  varying  from  20°  to  27'^.  The  prices  of 
these  prisms  vary  with  the  size  and  purity  of  the  crystals. 

103.  Common  and  Polarized  liiglit  Contrasted. 


A Ray  of  Common  Lights 

1.  Is  capable  of  reflection  at  oblique 
angles  of  incidence,  in  every  position  of 
the  reflector. 

2.  Penetrates  a plate  of  tourmaline 
(cut  parallel  to  the  axis  of  the  erystal) 
in  every  position  of  the  plate. 

3.  Penetrates  a bundle  of  parallel  glass 
plates,  in  every  position  of  the  bundle. 

4.  Suffers  double  refraction  by  Iceland 
spar,  in  every  direction  except  that 
parallel  to  the  major  axis  of  the  crystal. 


A Ray  of  Polarized  Lights 

1.  Is  capable  of  reflection  at  oblique 
angles  of  incidence,  in  certain  positions 
only  of  the  reflector. 

2.  Penetrates  a plate  of  tourmaline 
(cut  parallel  to  the  axis  of  the  crystal) 
in  certain  positions  of  the  plate,  but  in 
others  it  is  wholly  intercepted. 

3.  Penetrates  a bundle  of  parallel 
glass  plates  in  certain  positions  of  the 
bundle,  but  not  in  others. 

4.  Does  not  suffer  double  refraction 
by  Iceland  spar  in  every  direction,  not 
parallel  to  the  major  axis  of  the  crystal. 
In  some  other  positions  it  suffers  single 
refraction  only. 


THEORY  OF  POLARIZED  LIGHT. 

lOT.  CiidHlatory  Theory.  In  attempting  to  account  for 
the  phenomena  of  polarized  light,  the  most  satisfactory  expla- 
nations are  furnished  by  the  undulatory  theory  of  light,  first 
proposed  by  Huyghens  and  more  fully  investigated  by  Dr. 
Thomas  Young. 

According  to  this  theory,  the  particles  of  luminous  bodies 


* The  refractive  index  of  Iceland  spar,  for  the  extraordinary  ray,  varies  by  a 
somewhat  complicated  law,  ranging  from  1.483  to  1.654,  but  for  such  pencils  of 
light  as  can  be  transmitted  by  Nicol’s  prism,  it  varies  only  between  1.5  and  1.56. 
The  extraordinary  ray  in  this  prism  never  suffers  total  reflection  by  the  balsam, 
unless  it  approaches  within  about  10°  of  its  surface. 


CATALOGUE  OF  ACHROMATIC  MICROSCOPES. 


80  POLARIZED  LIGHT  AND  ITS  APPLICATION  TO  THE  MICROSCOPE. 


are  constantly  in  a state  of  alternate  contraction  and  expansion, 
and  are  capable  of  communicating  tlieir  own  motion  to  a very 
subtle  ether  pervading  universal  space,  (and  even  solid  bodies 
themselves,)  through  which  vibrations  are  propagated  like 
waves  through  water,  but  with  immensely  greater  velocity. 

105.  Illustrations  of  wave  motion.  Let  a rope,  made 
fast  at  one  end,  be  stretched  in  a horizontal  direction,  while  the 
other  end  is  held  in  the  hand ; if  this  end  is  moved  quickly 
upwards  and  downwards  at  regular  intervals,  an  undulatory 
motion  will  be  propagated  through  every  part  of  the  rope,  by 
a series  of  tremors  or  waves  passing  along  from  one  end  to  the 
other.  The  vibratory  motions  will  all  take  place  in  a perpen- 
dicular plane,  the  motion  of  each  particle  being  at  right  angles 
to  the  general  direction  of  the  rope.  This  vibration  of  the 
rope  may  represent  the  vibrations  of  one  of  the  polarized  rays 
into  which  common  light  is  separated  by  double  refraction. 

Let  another  similar  rope  be  agitated  in  the  same  manner 
from  right  to  left,  the  particles  of  this  latter  rope  will  vibrate 
in  a plane  perpendicular  to  the  vibrations  of  the  other  rope, 
and  will  serve  to  illustrate  the  vibrations  of  the  other  ray  of 
light,  polarized  at  right  angles  to  the  first,  by  the  action  of  the 
doubly  refracting  medium. 

If  we  suppose  a single  rope  agitated  successively  in  an  infi- 
nite number  of  planes,  varying  through  every  degree  of  the 
circle,  the  diflferently  inclined  vibrations,  following  each  other 
at  infinitely  short  (but  successive)  intervals,  while  the  vibra- 
tions would  take  place  in  every  possible  plane,  the  successive 
waves,  by  which  the  vibrations  would  be  propagated,  would 
advance  like  the  coils  of  a spiral,  or  the  threads  of  a screw. 

Let  us  suppose  this  rope,  whose  waves  are  propagated  in  a 
spiral  direction,  gradually  restrained  by  the  approach  of  two 
plane  resisting  surfaces,  parallel  to  each  other,  the  spiral  motion 
would  be  gradually  obliterated,  and  vibration  would  be  con- 
tinued only  in  a single  plane,  as  was  supposed  in  the  case 
of  the  first  rope.  This  may  serve  to  illustrate,  somewhat,  the 
action  of  the  tourmaline,  which  transmits  light  polarized  or 
vibrating  only  in  a single  plane. 


J.  & W.  GRUNOW  & GO’S  ILLUSTRATED 


POLARIZATION  ILLUSTRATED  BY  RESULTANNT  MOTIONS.  81 


106.  Polarization  illustrated  by  resultant  motions. 

Every  student  of  meclianics  knows,  that  two  forces  at  right 
angles  to  each  other  may  combine  and  form  a resultant  force 
represented  in  direction  and  intensity  by  the  diagonal  of  a 
parallelogram,  the  sides  of  which  represent  the  direction  and  in- 
tensity of  the  original  forces ; and  that  a single  force,  repre- 
sented in  direction  and  intensity  by  the  diagonal  of  a parallel- 
ogram, may  be  resolved  into  two  forces  at  right  angles  to  each 
other,  which  will  be  represented  in  direction  and  intensity  by 
the  sides  of  the  parallelogram. 

Applying  these  principles 
to  illustrate  the  polarization 
of  light,  let  O,  Fig.  43, 
represent  the  centre  or  axis 
of  a ray  of  common  light 
passing  in  a direction  per- 
pendicular to  the  plane  of 
the  paper.  Let  A B,  G H, 

D C,  F E and  I J,  represent 
transverse  sections  of  the 
planes  in  every  direction  in 
which  the  ray  of  light  causes 
the  luminiferous  medium  to 
vibrate,  we  can  always  se- 
lect two  planes,  as  A B,  C D,  at  right  angles  to  each  other, 
which  shall  correspond  with  the  planes  of  polarization  in  which 
the  light  vibrates  after  double  refraction.  The  vibrations  in  all 
the  other  planes,  in  which  ordinary  light  is  supposed  to  vibrate, 
may  be  resolved  into  vibrations  in  the  planes  A B and  C D. 
Thus  the  vibration  O G will  be  equivalent  to  two  vibrations 
represented  by  O a and  O d ; OF  will  be  equivalent  to  O 5 
and  O d'  j O H will  be  equivalent  to  O V and  O cy  O E will 
be  equivalent  to  O aJ  and  O g\  and  so  on.  AV  can  thus  resolve 
the  vibrations  in  any  number  of  planes  into  others  in  the 
planes  A B and  C D. 

Vibrations  O I,  very  nearly  coinciding  with  one  of  the  planes 
C D,  will  give  a resultant  intensity  in  the  direction  of  that 


CATALOGUE  OF  ACHROMATIC  MICROSCOPES. 
6 


82  POL  SEIZED  LIGHT  AND  ITS  APPLICATION  TO  THE  MICKOSCOPE. 


polarizing  plane,  almost  equal  to  the  original  intensity,  and  but 
a feeble  intensity  in  the  other  polarizing  plane.  But  there  will 
also  be  vibrations  very  nearly  coinciding  with  each  of  the  polari- 
zing planes,  so  that  the  sum  of  the  resulting  vibrations  in 
each  polarizing  plane,  will  be  exactly  equal. 

The  rays  polarized  in  each  plane  will  be  represented  in  inten- 
sity by  the  sum  of  all  the  resultants  in  that  plane. 

Figure  44  is  designed  to  represent  a transverse  section  of  a 
ray  of  common  light,  vibrating  in  an  infinite  number  of  planes, 
and  at  right  angles  to  the  direction  of  the  ray. 


Fig.  44.  Fig.  45.  Fig.  46. 


Figure  45  is  designed  to  represent  this  vibrating  ray,  resolved 
into  vibrations  in  two  directions  at  right  angles  to  each  other, 
as  when  a ray  of  common  light  undergoes  double  refraction 
and  polarization  by  passing  through  Iceland  spar. 

Figure  46  shows  another  and  more  common  method  of  repre- 
senting the  two  rays,  produced  by  double  refraction,  polarized 
in  the  planes  A B and  C D,  the  fine  lines  in  A B and  C D,  in- 
dicating the  condensation  and  combination  of  resultant  vibra- 
tions, entirely  separated  from  the  vibrations  in  the  other  plane 
of  polarization. 

107.  Polarizing  effect  of  Iceland  Spar.  Thus  tlie  polari- 
zing action  of  Iceland  spar,  and  all  doubly  refracting  substan- 
ces, is  to  separate  a ray  of  common  light,  whose  vibrations  are 
in  every  plane  passing  through  the  direction  of  the  ray,  into 
two  parallel  polarized  rays,  whose  vibrations  are  in  planes  at 
right  angles  to  each  other. 

The  preceding  illustrations  may  aid  in  understanding  that  if 
a 'polarized  ray  falls  upon  another  polarizing  medium,  in 


J.  & W.  GRUNOW  & GO’S  ILLUSTRATED 


PARTIAL  POLARIZATION. 


83 


such  a position  that  its  vibrations  are  oblique  to  the  polarizing 
planes  or  axes  of  the  medium,  they  will  be  resolved  into  vibra- 
tions in  hoth  those  axes  or  polarizing  planes,  and  two  new 
polarized  rays  will  result,  each  of  which  might  be  again  sub- 
divided in  the  same  manner  by  another  polarizing  prism  placed 
in  a position  oblique  to  the  new  axes. 

108.  Familiar  illustrations.  A ray  of  common  light  is 
sometimes  compared  to  a cylindrical  rod,  whereas  the  polarized 
rays  are  like  two  flat  parallel  rulers,  one  of  which  is  laid  hori- 
zontally on  its  broad  suface,  and  the  other  horizontally  on  its 
edge.  The  alternate  transmission  and  obstruction  of  one  of  the 
flattened  beams,  by  the  tourmaline,  is  similar  to  the  facility 
with  which  a card,  or  flat  ruler,  may  be  passed  between  the 
wires  of  a cage  if  presented  edgewise,  and  the  impossibility  of 
its  passing  in  a transverse  position. 

We  may  also  suppose  a refracting  substance,  with  a reflect- 
ing surface,  made  up  of  parallel  fibres.  Such  a surface  would 
allow  the  passage  of  all  the  rays  in  common  light  which  vibrate 
in  a plane  parallel  to  the  direction  of  its  fibres  and  would 
reflect  the  rest ; while  polarized  light,  if  vibrating  in  a plane 
parallel  to  the  fibres,  would  be  wholly  transmitted,  but  if  vibra- 
ting in  a plane  at  right  angles  to  the  fibres,  it  would  be  wholly 
reflected. 

109.  Partial  Polarization.  Having  already  explained  in 
the  previous  section  how  light  is  polarized,  1st,  by  reflection  ; 
2nd,  by  refraction ; 3rd,  by  transmission  through  bundles  of 
thin  plates  ; and  4th,  by  double  refraction  ; it  now  remains  to 
state  that  a great  variety  of  substances,  and  in  different  condi- 
tions, ^xodiWQQ  partial  polarization  of  light  reflected  from  their 
surfaces  or  transmitted  through  them. 

It  is  well  known  that  no  substance  either  transmits^  or  reflects^ 
all  the  light  that  falls  upon  it ; even  the  most  transparent  sub- 
stances reflect  a small  portion  of  light,  the  proportion  reflected 
and  transmitted  varying  with  every  angle  of  incidence.  While 
plate  glass  polarizes  very  nearly  all  the  light  that  falls  upon  it 
at  an  angle  of  57°,  the  intensity  of  the  reflected  ray  is  equal  to 
only  one-half  the  intensity  of  the  incident  ray,  the  other  half 
of  the  light  is  transmitted  through  the  glass,  and  is,  at  the 


CATALOGUE  OF  ACHROMATIC  MICROSCOPES. 


84  POLARIZED  LIGHT  AND  ITS  APPLICATION  TO  THE  MICROSCOPE. 


same  time,  polarized  by  refraction  in  a plane  at  right  angles 
with  the  plane  of  the  reflected  ray.  If  the  light  falls  npon  the 
glass  at  any  other  angle,  some  portion  of  it  will  be  polarized  by 
reflection,  and  an  equal  amount  will  be  polarized  by  refraction, 
but  each  of  the  rays  thus  polarized  will  be  mingled  with  other 
light  that  is  reflected  or  transmitted  without  polarization. 

If  instead  of  transmitting  light  through  a bundle  of  plates  of 
thin  glass,  we  transmit  it  through  only  a single  plate,  and  at 
any  angle,  that  plate  will  polarize  a part  of  the  light ; if  two 
plates  are  used,  still  more  light  will  be  polarized,  and  when  a 
sufficient  number  of  plates  are  used,  all  the  light  will  be 
polarized. 

Many  substances,  not  perfectly  homogeneous  throughout, 
have  veins,  laminae,  or  isolated  spots,  capable  of  producing 
either  partial  or  complete  polarization  of  the  light  which 
passes  through  them. 

In  Iceland  spar  the  two  images  produced  by  double  refrac- 
tion are  equal  in  intensity,  and  are  both  completely  polarized  ; 
but  a great  variety  of  substances  give,  by  refraction,  a secon- 
dary image  so  faint  and  so  little  separated  from  the  ordinary 
image,  that  its  existence  is  not  generally  recognized  unless 
examined  by  some  test  for  polarized  light.  There  are  many 
substances,  both  animal  and  vegetable,  that  possess  this  power 
of  partial  polarization,  and  which,  when  viewed  by  polarized 
light,  exhibit  an  arrangement  of  structure  totally  un apprecia- 
ble by  ordinary  light. 

Even  glass  which  has  been  cooled  more  rapidly  in  one  part 
than  another,  or  in  which  unequal  pressure  has  been  exerted 
in  any  manner,  (as  by  bending  or  heating  on  one  side,)  is 
capable  of  polarizing  some  of  the  light  transmitted  through  it. 
Many  crystals  found  in  animal  fluids  or  vegetable  tissues,  pos- 
sess the  power  of  partially  polarizing  light.  Hence,  polarized 
light  becomes  the  most  delicate  test  known  for  discovering 
differences  of  density  or  structure,  in  animal,  vegetable,  or  min- 
eral substances,  and  is  of  great  importance  as  a method  of  illu- 
mination in  microscopic  investigations. 


J.  & W.  GRUNOW  & GO’S  ILLUSTRATED 


POLARIZED  LIGHT  APPLIED  TO  THE  MICROSCOPE. 


85 


POLARIZED  LIGHT  APPLIED  TO  THE  MICROSCOPE. 

110.  Polarizing  apparatus.  The  apparatus  employed  for 
microscopic  investigations  with  polarized  light,  consist  of  a 
Nicol’s  prism  placed  below  the  stage  and  called  jpolarizeT 
and  an  analyzer^  which  is  usually  also  a Nicol’s  prism,  set  in  a 
brass  tube  and  inserted  in  the  body  of  the  microscope  imme- 
diately behind  the  object-glass. 

The  'polarizer  is  so  mounted  that  it  can  be  revolved,  allowing 
the  polarizing  planes  of  the  two  prisms  to  be  made  to  coincide 
or  to  make  with  each  other  any  required  angle.  These  two  in- 
struments together  constitute  the  Polariscojpe, 


Fig.  47.  Fig.  48. 


MOUNTING  OF  POLARIZING  PRISM.  MOUNTING  OF  ANALYZING  PRISM. 

Figure  47  shows  the  mounting  of  the  polarizer  which  is 
attached  to  the  stage  by  a bayonet  joint.  The  polarizing  prism 
is  revolved  by  turning  the  milled  head  at  the  bottom  of  the 
instrument. 

Figure  48  shows  the  mounting  of  the  analyzer,  which  is  in- 
serted in  the  lower  end  of  the  compound  body,  and  attached  by 
a bayonet-joint  in  the  usual  position  of  the  object-glass,  but 
passing  upward  into  the  body,  the  object-glass  being  attached 
to  the  lower  end  of  the  analyzer. 

Figure  49  shows  a section  of  the  polarizer  and  the  form  of 
mounting.  Above  and  below  the  ISTicol’s  prism  are  circles  of 
thin  glass  to  protect  the  delicate  faces  of  the  prism  from  injury. 


CATALOGUE  OF  ACHROMATIC  MICROSCOPES. 


86  POLARIZED  LIGHT  AND  ITS  APPLICATION  TO  THE  MICROSCOPE. 


Figure  50  shows  a section  of  the  analyzer  consisting  of  a 
Nicol’s  prism  and  its  mounting,  and  protected  like  the  polarizer 
by  circles  of  thin  glass. 


Fig.  60. 


SECTION  OF  POLARIZER. 


SECTION  OP  ANALYZER. 


The  mountings  of  the  jpolariscojpe  are  made  of  different 
sizes,  to  suit  both  large  and  small  microscopes,  and  the  prices 
vary  accordingly,  depending  principally  upon  the  size  and 
quality  of  the  i^icol’s  prisms  employed.  The  largest  and  best 
Nicol’s  prisms  transmit  the  most  light  and  are  suited  for  the 
more  delicate  investigations. 

Some  opticians  place  the  analyzer  above  the  eye-piece,  but 
this  arrangement  diminishes  the  extent  of  the  field.  If  the 
analyzing  prism  is  small,  and  is  placed  just  above  the  object- 
glass,  it  stops  out  a portion  of  the  light,  but  if  it  is  of  sufficient 
size  it  transmits  all  the  light  from  the  object-glass,  and  does  not 
in  any  manner  limit  the  field  of  view.  For  these  reasons  we 
prefer  mounting  the  analyzer  in  the  body  of  the  microscope 
just  above  the  object-glass.  It  may  also  be  adapted  to  the 
lower  end  of  the  draw-tube,  which  arrangement  allows  it  to  be 
rotated  by  the  milled  ring  of  the  tube  itself. 

111.  A Touriiialiac  plate  fitted  to  the  Eye-piece  makes 
an  excellent  analyzer,  when  it  can  be  obtained  of  suitable  size 
and  purity  of  color.  Such  tourmaline  plates  are  compara- 
tively rare  and  costly. 


J.  & W.  GRUNOW  & GO’S  ILLUSTRATED 


COLORED  POLARIZATION. 


87 


112.  IIerai>atBiite  or  Artificial  Tourii(ialiiac§.  These 
are  crystals  of  disulphate  of  iodine  and  quinine^  and  when  of 
sufficient  size  can  be  used  both  as  polarizers  and  analyzers. 
They  are  called  Herapathite  from  the  name  of  their  discoverer, 
Dr.  Herapath,  but  large  crystals  have  not  yet  been  produced 
in  sufficient  abundance  to  allow  of  their  general  use. 

113.  Valite  of  the  Polariscope  isi  Micro§copic  iaivesti- 
gatioifi§.  When  the  polariscope  is  attached  to  the  microscope, 
the  polarizer  being  below  the  stage  and  the  analyzer  above  it, 
any  object  can  be  subjected  to  examination  with  polarized 
light.  Objects  having  parts  of  their  structure  more  dense  than 
others,  will  present  greater  contrast  of  light  and  shade  than  by 
ordinary  light,  and  thus  the  most  delicate  structures,  as  capil- 
lary blood-vessels,  nerves,  cell-walls,  &c.,  will  be  well  defined 
where  they  could  not  be  otherwise  distinguished.  Sections  of 
horn,  teeth,  bones,  quills,  shells,  and  many  vegetable  tissues, 
exhibit  their  delicate  structure  under  the  influence  of  polarized 
light.  Revolving  the  polarizer  causes  the  relative  brightness  of 
difierent  structures  to  vary,  and  is  essential  in  developing  the 
greatest  effect  of  polarized  light  in  distinguishing  delicate 
structures.  Those  portions  of  an  object,  possessing  the  power 
of  partial  or  complete  polarization,  act  like  particles  or  veins  of 
tourmaline  to  obstruct  the  passage  of  polarized  light  in  certain 
positions,  while  the  other  parts  of  the  object  appear  as  lumin- 
ous as  by  ordinary  light.  So,  also,  when  the  position  of  the 
polarizer  and  analyzer  is  such  as  to  cut  off  all  the  light  from 
ordinary  objects,  the  delicate  structures  that  possess  the  property 
of  polarizing  light,  depolarize  the  light  already  polarized  and 
allow  it  to  be  transmitted,  showing  points  or  veins  brilliantly 
illuminated  amid  other  parts  of  the  object  which  appear  dark. 

It  should  therefore  be  laid  down  as  a rule  in  microscopic  in- 
vestigation, says  Prof.  Queckett,  “ That  every  new  variety  of 
tissue  should  be  subjected  to  the  action  of  polarized  light.” 

114.  Colored  Polarization.  All  substances,  whether  ani- 
mal, vegetable,  or  mineral,  which,  by  the  unequal  arrangement 
of  their  particles  possess  the  property  of  double  refraction, 
when  placed  between  the  polarizing  and  analyzing  prisms 
exhibit  colors^  varying  according  to  the  otherwise  unapprecia- 


CATALOGUE  OF  ACHROMATIC  MICROSCOPES. 


88  POLAKIZED  LIGHT  AND  ITS  APPLICATION  TO  THE  MICEOSCOPE. 


ble  difference  of  density  of  their  various  parts,  and  these  differ- 
ences may  thus  be  distinguished  and  traced  out  much  more 
satisfactorily  than  by  common  light.  Polarized  light  may  be 
compared  to  a new  sense  given  to  the  student  of  nature,  by 
which  he  is  enabled  to  see  things  wholly  invisible  by  ordinary 
light. 

Where  the  doubly-refracting  properties  of  the  tissue  are  too 
feeble  to  produce  sufficient  difference  of  color,  the  effect  may 
be  increased  by  placing  the  object  over  a plate  of  selenite  or 
mica,  of  such  a thickness  as  to  give  to  the  light  any  shade  of 
color  required, 

115.  Tiie  Cawse  of  Colors  produced  toy  selenite  or  mica, 
when  polarized  light  is  transmitted  through  them,  is  easily 
understood  by  reference  to  the  undulatory  theory  of  light.  In 
all  doubly  refracting  substances,  (of  which  selenite  and  mica 
are  examples,)  the  ordinary  and  extraordinary  rays  move  with 
different  velocities,  and  consequently,  when  the  two  rays  are 
again  blended,  unless  the  retardation  amounts'  to  a certain  num- 
ber of  entire  waves,  the  two  rays  will,  by  the  interference  of 
their  waves,  produce  some  change  in  the  color  of  light.  If  the 
retardation  equals  any  number  of  entire  vibrations,  the  result 
will  still  be  white  light,  the  two  rays  conspiring  to  increase 
their  mutual  intensity.  If  one  ray  is  retarded  an  odd  number 
of  half  vibrations,  they  will  mutually  destroy  each  other  and 
produce  darkness,  just  as  if  two  waves  of  the  sea  meet  in  such 
a state  that  the  phase  of  elevation  in  one  coincides  with  the 
phase  of  depression  in  another,  the  two  will  produce  a level,  or 
mutual  obliteration  results.  Such  a result  in  the  case  of  light, 
would  require  the  most  exact  adjustment  of  the  thickness  of 
the  crystal,  and  would  not  often  occur. 

The  interference  produced  by  selenite  and  mica,  are,  in 
general,  similar  to  the  results  which  would  be  obtained  by 
placing  one  prismatic  spectrum  upon  another,  in  a reverse 
position,  but  not  exactly  superimposed  upon  it.  The  amount 
of  overlaping  would  determine  the  resultant  color. 

116.  Meltood  Varying  llie  CoSors.  When  a film  of 
selenite,  of  uniform  thickness,  is  placed  between  the  polarizer 


J.  & W.  GRUNOW  k CO’S  ILLUSTRATED 


ISIETHOD  OF  VARYINO  THE  COLORS. 


89 


and  analyzer,  on  rotating  tlie  film  a brilliant  color  is  perceived 
at  every  quadrant  of  a circle,  but  in  intermediate  positions  it 
vanishes  altogether.  We  observe,  however,  that  the  tint  does 
not  change,  but  only  varies  in  intensity. 

If,  now,  the  film  of  selenite  is  fixed  and  the  polarizer  is 
rotated,  we  also  observe  color  at  every  quadrant  of  revolution 
wdiich  disappears  in  intermediate  positions,  but  the  tint  changes 
and  becomes  complementary  at  every  quadrant, — the  same  tint 
reappearing  at  every  half-revolution.  Thus,  when  the  film 
alone  is  revolved,  one  color  only  is  seen,  but  when  the  polarizer 
is  revolved  two  complementary  colors  are  seen. 

The  following  is  Sir  David  Brewster’s  table  of  complemen- 
tary colors : 


Red, 

complementary. 

Bluish  green. 

Orange, 

Blue, 

Yellow, 

Indigo, 

Green, 

Violet  reddish. 

Blue, 

Orange  red. 

Indigo, 

Orange  yellow, 

Violet, 

a 

Yellow  green. 

Black, 

White, 

White, 

Black. 

Films  of  selenite,  varying  between  .00124  and  .01818  of  an 
inch  in  thickness,  will  give  every  variety  of  tint  in  the  solar 
spectrum.  If  two  films  of  selenite  are  placed  over  each  other, 
with  their  crystallographic  axes  parallel,  the  color  produced 
wdll  be  that  which  belongs  to  the  sum  of  their  thicknesses. 
But  when  the  two  films  are  placed  with  similar  axes  at  right 
angles,  the  resulting  tint  is  that  which  belongs  to  the  difference 
of  their  thicknesses. 

A film  of  selenite  or  mica  of  such  thickness  as  to  produce  a 
bright  purple,  or  a light  blue  color,  will  be  found  to  present  the 
most  agreeable  contrast,  and,  as  a single  plate,  prove  most 
generally  useful  to  the  microscopist.  Three  films  of  selenite, 
which  separately  give  three  different  colors,  may  each  be 
mounted  in  Canada  balsam,  between  slips  of  thin  glass,  and 
used  singly,  or  in  double,  or  triple  combinations.  As  many  as 
thirteen  different  tints  may  thus  be  obtained. 


CATALOGUE  OF  ACHROMATIC  MICROSCOPES. 


90  POLARIZED  LIGHT  AND  ITS  APPLICATION  TO  THE  MICROSCOPE. 


117.  Seleaiite  §tage.  This  instrument,  which  was  invented 
by  Mr.  Darker,  is  shown  in  Fig.  51.  It  consists  of  a plate  of 
brass,  three  or  four  inches  long,  1^  inches  broad,  and  J of  an 
inch  thick,  having  a piece  of  raised  brass  screwed  to  it,  against 
which  objects  may  rest  when  the  body  of  the  microscope  is 
inclined. 


Fig.  51. 


In  the  centre  of  the  brass  plate  there  is  a hole,  one  inch  in 
diameter,  into  which  is  fitted  a ring  of  the  same  metal,  with  a 
shoulder  on  its  under  side  to  receive  certain  cells,  into  which 
plates  of  selenite  are  fitted ; this  ring  can  be  revolved  either  to 
the  right  or  the  left  of  a central  index  or  dart,  by  means  of  an 
endless  screw  S.  P A J,  P A |,  P A represent  three  brass 
cells,  into  each  of  which  are  burnished  two  plates  of  thin  glass^ 
having  between  them  films  of  selenite  of  difierent  thicknesses. 
The  dart  P A,  denotes  the  direction  of  the  positive  axis  of  the 
selenite,  and  the  figures  •},  f , J,  denote  the  parts  of  a vibration 
retarded  by  each  disc,  which,  by  their  superposition  and  varia- 
tion in  position,  by  means  of  the  endless  screw  motion,  produce 
all  the  colors  of  the  spectrum. 

118.  Polarizer  with  RevoSviaig  §eleiiiitc  Carrier.  In 

order  to  afford  the  greatest  facility  of  revolving  the  selenite 
plate,  and  for  convenience  of  using  it,  a revolving  selenite 
carrier  is  attached  to  the  polarizer,  as  shown  in  Fig.  52.  The 
solid  ring  is  attached  to  the  stage  in  the  usual  way  by  a 
bayonet-joint.  A cylinder,  with  a milled  head  c,  is  supported 
by  the  ring  d,  which  revolves  in  the  ring  a.  Upon  d rests  the 
selenite  carrier  5,  covered  by  the  cap  e,  so  that  the  selenite  plate 
is  revolved  by  turning  the  milled  head  c.  Within  the  cylinder 


J.  & W.  GRUNOW  & GO’S  ILLUSTRATED 


DELICATE  STRUCTURES,  VIEWED  BY  COLORED  POLARIZED  LIGHT.  91 


v/hich  supports  the  selenite,  another  cylinder,  carrying  the 
Nicol’s  prism  P,  is  separately  revolved  by  turning  the  milled 
b.  The  Nicol’s  prism  revolves 
more  easily  than  the  selenite 
carrier,  so  that  the  latter  may 
remain  stationary  when  the 
former  is  revolved.  By  turn- 
ing the  milled  head  c,  the 
polarizer  and  selenite  are  re- 
volved together.  This  has  the 
same  effect  upon  the  light  as  if 
the  analyzer  were  revolved. 

By  means  of  this  apparatus, 
the  axes  of  the  polarizing  prism, 
selenite  plate,  and  analyzer^ 
may  be  placed  in  any  relative 
position  desired,  affording  great  facility  for  using  polarized 
light  colored  of  any  tint  required  in  microscopic  investigations. 
We  are  happy  to  acknowledge  our  obligations  to  Dr.  White 
of  'New  Haven,  for  suggesting  this  improved  arrangement  for 
revolving  the  plate  of  selenite. 

119.  Delicate  structures,  viewed  by  colored  polarized 
light,  produce  much  more  sensible  changes  upon  the  light  than 
upon  plain  polarized  lights — microscopic  crystals  are  especially 
beautiful  when  viewed  in  this  manner.  The  crystallization  of 
various  salts,  viewed  by  polarized  light,  is  a subject  of  great 
importance  to  the  practical  chemist  and  mineralogist.  So 
minute  a quantity  as  one  thirteen  millionth  of  a grain  of 
potassa,  when  tested  with  bichloride  of  platinum,  gives  a dis- 
tinct and  characteristic  tint,  sufficient  to  distinguish  it  from 
every  other  alkali,  when  viewed  in  the  microscope  by  this 
kind  of  light.  Many  substances,  known  in  organic  chemistry, 
are  more  readily  distinguished  by  polarized  light  than  by  any 
other  means. 

The  examination  of  microscopic  structures,  by  polarized 
light,  affords  to  the  enterprising  student  a rich  field  of  investi- 
gation, as  yet  but  partially  explored.  As  a method  of  investi- 
gating delicate  structures  it  is  of  the  highest  value.  The  chem- 


CATALOGUE  OF  ACHROMATIC  MICROSCOPES. 


92  POLAEIZED  LIGHT  AND  ITS  APPLICATION  TO  THE  MICEOSCOPE. 


ist  may  perform  the  most  dexterous  analysis  ; the  crystallogra- 
pher  may  examine  crystals  by  the  nicest  determination  of  their 
forms  and  cleavage ; the  anatomist  or  botanist  may  use  the  dis- 
secting knife  and  microscope  with  the  most  exquisite  skill ; but 
there  are  still  structures  in  the  mineral,  vegetable,  and  animal 
kingdoms,  which  defy  all  such  modes  of  examination,  and 
which  yield  only  to  the  magical  analysis  of  polarized  light.  A 
body,  which  is  quite  transparent  to  the  eye,  and  which  might 
be  judged  as  monotonous  in  structure  as  it  is  in  aspect,  will  yet 
exhibit,  under  polarized  light,  the  most  exquisite  organization, 
and  will  display  the  result  of  new  laws  of  combination  which 
the  imagination  even  could  scarcely  have  conceived. 


120.  Iii§t  off  Otojects  for  tlie  Polariscope.  Sufficient 
having  now  been  stated  to  give  the  reader  a general  view  of 
the  nature  and  use  of  polarized  light  and  its  application  to  the 
microscope,  we  shall  conclude  this  subject  by  simply  giving  a 

TABLE  OF  THE  MOEE  INTEEESTING  OBJECTS  FOE  THE  MICEOSCOPE, 
WHICH  AEE  ESPECIALLY  BEAUTIFUL  WITH  POLAEIZED  LIGHT  : 


ANIMAL  STRUCTUKES. 


^ Bone  of  Cuttle-fish, 

^ Fibres  of  Sponge, 

Hoof  of  Ass, 

Hoof  of  Camel, 

Hoof  of  Sheep, 

Hoof  of  Horse, 

Hoof  of  Ox, 

Horn  of  African  Rhinoceros, 

transverse  section, 
longitudinal  section, 
Horn  of  Indian  Rhinoceros, 

Horn  of  Antelope, 

Horn  of  Ox, 

Horn  of  Sheep, 

Polyzoaries, 
y Quill  of  Porcupine, 

Quill  of  Echidna, 

Quill  of  Condor, 

Tendon,  Human, 

Tendon,  Ostrich, 

Gray  Human  Hair, 


Raw  Silk, 

Scale  of  Eel, 

Scale  of  Sole, 

Skin,  Elephant, 

Skin,  Crocodile, 

Skin,  Human,  f- 
Skin,  Rhinoceros, 

Skin,  various  Serpents, 
Spicules  of  Gorgonia,  r* 
Whalebone, 

Palate  of  Whelk,  ^ 
Palate  of  Limpet, 

Palate  of  Nassa, 

Palate  of  Paludina, 
Palate  of  Cyclostoma, 
Wing  cases  of  Beetles, 
Scales  of  Fishes, 

Sections  of  Hairs,  P* 

Sections  of  Teeth,  ^ 
Nerves  and  Muscle. 


J.  k W.  GRUNOW  & GO’S  ILLUSTRATED 


LIST  OF  OBJECTS  FOR  THE  POLARISCOPE. 


93 


VEGETABLE. 


y Starch,  Potato, 

X Starch,  Arrowroot, 

Starch,  Custard -powder, 

X Starch,  Indian-corn, 

Starch,  Tous  les  Mois, 
Gun-cotton, 

y Hairs  from  leaf  of  Deutzia, 
i Hairs  from  leaf  of  Elaeagnus, 
Hairs  from  leaf  of  Olive, 
Hairs  from  leaf  of  Cactus, 


Raw  Cotton,  ^ 

Raw  Flax,  ^ 

Siliceous  Cuticle  of  Bamboo, 
Siliceous  Cuticle  of  Eqiiisetum, 
Siliceous  Cuticle  of  Rice, 

Siliceous  Cuticle  of  Wheat, 
Raphides, 

Spiral  Cells  and  Vessels, 

Wood,  longitudinal  sections  mount- 
ed in  balsam. 


y Agate, 
Arragonite, 

X Asbestos, 
Adventurine, 
Granite, 
Marble, 

y Selenite  films. 


MINERAL. 

Sea  sand, 

Tremolite, 

Satin  Spar, 

Sandstone^ 

Feldspar, 

Crystals  (of  Titanic  Iron,  ?)  in  some 
varieties  of  mica. 


CRYSTALS,  VIZ: 


Acetate  of  Copper, 
Bichromate  of  Potash, 
y Borax, 

Boracic  Acid, 

Borate  of  Ammonia, 

Borax  and  Phosphoric  Acid, 
Carbonate  of  Lime, 
Chromate  of  Potash, 
Chlorate  of  Potash, 
Cholesterine, 

Nitric  Acid, 

Epson  Salts, 

Oxalic  Acid, 

Oxalate  of  Ammonia, 
Oxalate  of  Chromium, 
Oxalate  of  Lime, 

Oxalate  of  Soda, 


Nitrate  of  Ammonia, 
Nitrate  of  Baryta, 

Nitrate  of  Lead, 

Nitrate  of  Potash, 

Nitrate  of  Soda, 
Phosphate  of  Soda, 
Salicine,  % 

Sugar, 

Sugar  of  Milk, 

Sulphate  of  Cadmium, 
Sulphate  of  Copper, 
Sulphate  of  Magnesia, 
Sulphate  of  Nickel, 
Tartaric  Acid,  ' 

Tartrate  of  Lime, 

Uric  (or  Lithic)  Acid,  ^ 
Triple  Phosphate. 


CATALOGUE  OF  ACHROMATIC  MICROSCOPES. 


94 


PRACTICAL  DIRECTIONS. 


CHAPTER  V. 

PRACTICAL  DIRECTIONS. 

121.  Care  of  tlie  ]TIicro§cope.  It  is  of  the  first  importance 
that  every  part  of  the  microscope  should  be  kept  perfectly 
clean  and  free  from  dust.  For  this  purpose  a good  case  is  re- 
quired, into  which  the  instrument  can  be  easily  placed  wdien 
not  in  use.  In  order  that  no  unnecessary  time  may  be  lost  in 
packing  and  unpacking,  the  upright  case  is  more  convenient, 
as  the  instrument  can  be  placed  in  it  nearly  in  the  same  condi- 
tion as  when  arranged  for  use.  Each  object-glass,  when  not  in 
use,  should  be  carefully  returned  to  the  brass  box  provided  for  it, 
as  it  is  thus  less  likely  to  be  injured  by  dust  or  other  means  than 
when  attached  to  the  microscope.  The  case  should  be  arranged 
with  proper  fittings  to  hold  all  the  apparatus  belonging  to  the 
microscope.  Two  or  three  drawers  are  required  to  hold  slides, 
thin  glass,  dissecting  needles,  &c. 

If  the  microscope  gets  soiled  it  may  be  wiped  with  fine  linen 
or  cambric.  The  lenses  of  the  eye-piece  may  be  wiped  wdth 
soft  buckskin  which  has  been  thoroughly  freed  from  dust,  but 
care  should  be  taken  that  this  operation  may  be  required  as 
seldom  as  possible.  The  object-glasses  will  lose  their  fine 
polish  if  often  wuped.  Some  microscopists  are  so  careful  of 
their  object-glasses  that  they  do  not  require  cleaning  for  years. 

If  an  object-glass  requires  cleaning  it  may  be  brushed  with 
a fine  short  carners-hair-pencil  which  is  used  for  no  other  pur- 
pose, or  it  may  be  moistened  by  breathing  on  it  and  then 
gently  pressed  with  soft  buckskin.  When  fluids  are  used 
about  the  microscope,  care  should  be  taken  to  avoid  wetting  the 
object-glasses.  If  this  accident  happens,  they  must  be  wiped 
dry  with  the  buckskin. 

In  handling  the  object-glasses,  care  should  be  taken  to  avoid 
touching  the  surface  of  the  lenses.  The  glasses  should  be 
examined  when  returned  to  the  case  to  see  if  they  have  been 
soiled.  Too  great  precaution  cannot  be  taken  to  keep  every- 
thing about  the  microscope  scrupulously  clean. 


J.  & W.  GRUNOW  & GO’S  ILLUSTRATED 


CHOICE  OF  A MICKOSCOPE. 


95 


122.  Illumination.  For  whatever  purpose  the  microscope 
may  be  used,  it  is  important  to  secure  a pure  and  adequate  illu- 
mination. The  young  microscopist  will  find  daylight  more 
easily  managed,  and  more  pleasant  for  the  eyes,  than  artificial 
light.  Even  experienced  microscopists  can  work  longer  and 
easier  by  good  daylight,  though  they  may  have  learned  to 
manage  artificial  light  so  as  to  make  it  more  available  than 
daylight  in  bad  weather. 

The  best  light  is  that  reflected  from  white  clouds,  but  light 
from  fleeting  clouds  is  troublesome  to  the  eyes  and  requires 
constant  moving  of  the  mirror.  Light  from  a luminous  atmos- 
phere, near  the  horizon,  is  better  than  that  near  the  zenith. 
The  direct  light  of  the  sun  cannot  ordinarily  be  used  for  illu- 
minating the  microscope,  but  the  light  of  the  sun,  reflected 
from  white  houses,  or  from  a white  plastered  wall,  gives  a soft 
and  beautiful  illumination. 

If  possible,  the  microscope  should  be  placed  at  a little  dis- 
tance from  a window  on  the  side  of  the  house,  opposite  to 
where  the  sun  is  shining.  If  artificial  light  is  used,  the  Ger- 
man student’s  lamp,  constructed  on  the  bird  fountain  principle, 
which  gives  excellent  illumination,  will  be  found  as  convenient 
as  any  other.  If  gas  light  is  used,  a pane  of  light  blue  glass 
placed  between  the  light  and  the  microscope,  will  take  off  the 
intense  glare  which  is  otherwise  apt  to  injure  the  eyes.  All 
flickering  lights  are  very  unpleasant  in  microscopic  investiga- 
tions. Observations  will  generally  be  conducted  with  greater 
ease,  when  the  body  of  the  microscope  is  inclined. 

123.  Ctioice  of  a Microscope.  In  choosing  a microscope, 
reference  should  always  be  paid  to  the  nature  of  the  investiga- 
tions for  which  it  is  to  be  used.  The  most  common  error 
among  inexperienced  persons,  is  the  idea  that  all  objects  can  be 
satisfactorily  examined  with  a high  magnifying  power.  This 
is  by  no  means  correct,  for  it  is  often  as  desirable  to  ascertain 
the  relations  of  different  portions  of  an  object,  as  to  examine 
minutely  any  single  part.  Small  crystals,  insects,  and  parts  of 
flowers,  hairs,  &c.,  which  are  readily  obtained,  are  best  studied 
with  magnifying  powers  of  from  twenty  to  one  hundred  diame- 
ters. Young  people  in  schools,  academies,  and  private  fami- 
lies, generally  first  examine  this  class  of  objects,  and  but  few 
such  persons  can  devote  the  requisite  attention  to  prepare  speci- 
mens with  sufficient  care  to  allow  of  their  being  examined  with 
magnifying  powers  higher  than  three  hundred  and  fifty 
diameters. 

The  Educational  Microscope,  (page  25  of  this  Catalogue,) 


CATALOGUE  OF  ACHROMATIC  MICROSCOPES. 


96 


PRACTICAL  DIRECTIONS. 


has  been  prepared  with  especial  reference  to  the  greatest  effi- 
ciency for  this  class  of  observers.  It  is  also  furnished  at  a 
very  moderate  price. 

Our  advice  is  often  solicited  by  persons  who  have  a limited 
amount  of  money  to  expend  for  a microscope,  and  who  desire 
to  know  what  selection  would  be  most  useful  for  their  several 
purposes.  For  those  who  design  to  devote  but  a limited  amount 
of  time  to  the  use  of  the  microscope,  we  would  recommend 
one  of  the  smaller  microscopes,  'No.  1,  2,  or  3,  with  the  appara- 
tus usually  attached  to  them  as  stated  in  our  Price  List. 

To  those  who  desire  to  have  at  once  an  instrument  suited 
to  the  more  delicate  investigations  at  the  smallest  cost,  we 
would  recommend  the  Student’s  Microscope,  Ho.  3,  furnish- 
ed with  First  Class  Ohjectives.^  to  be  purchased  according  to 
their  means  in  the  following  order:  For  the  study  of  botany 
and  mineralogy,  the  1 inch,  J inch,  ^ inch,  2 inch,  J inch, 
inch  objectives.  For  studying  entomology,  anatomy  and 
pathology,  the  -J-  inch,  i inch,  1 inch,  J inch,  2 inch,  ^2  i^^^h 
object-glasses. 

The  Student’s  Larger  Microscope,  Ho.  4,  furnished  with  1 
inch,  -J-  inch,  and  i inch  objectives,  Bull’s-eye  Condenser, 
Camera  Lucida,  and  Micrometer,  forms  a very  complete  instru- 
ment with  which  almost  all  vegetable  and  animal  tissues  and 
fluids  can  be  examined  satisfactorily.  This  combination  is 
especially  recommended  to  Medical  Students,  and  all  others 
who  intend  to  devote  considerable  attention  to  microscopic  in- 
vestigations, and  desire  to  make  the  most  economical  invest- 
ment of  limited  means,  and  yet  secure  s,  first  class  instrument. 

124.  equalities  of  Objcct-Crlasses.  In  considering  the 
value  of  an  object-glass,  and  its  adaptation  to  any  particular 
purpose,  several  distinct  qualities  are  to  be  examined,  viz:  (1.) 
its  defining  power  ^ or  the  power  of  giving  sharpness  of  outline, 
especially  on  the  borders  of  an  object,  or  where  dots  or  lines 
are  examined  ; (2.)  its  resolving  power.,  by  means  of  which 
closely-approximated  markings  are  distinguished ; (3.)  fiatness 
of  field  ; (4.)  depth  of  definition.,  which  refers  to  the  distance 
above  and  below  the  focus,  that  parts  of  an  object  can  be  seen 
with  tolerable  accuracy. 

1.  Defining  power ^ in  addition  to  perfect  workmanship,  depends  principally 
upon  the  perfection  of  the  corrections,  both  for  Spherical  and  for  Chromatic  aber- 
ration, The  difficulty  of  securing  this  perfect  correction  increases  with  the 
angular  aperture.  Any  inaccuracy  in  adjusting  the  object-glasses  for  thickness  of 
glass  cover,  which  every  observer  must  arrange  for  himself,  is  more  conspicuous 
in  glasses  of  large  angular  aperture.  For  this  reason,  object-glasses  of  moderate 


J.  & W.  GRUNOW  & CO’S  ILLUSTRATED 


QUALITIES  OF  OBJECT-GLASSES. 


97 


aperture  are  more  suitable  to  be  used  in  schools  and  private  families  where  many 
persons  use  the  same  microscope,  and  where,  for  want  of  time,  in  examining  a 
variety  of  specimens,  or  from  inexperience,  the  necessary  attention  cannot  be 
devoted  to  adjusting  the  correction  for  thickness  of  glass  cover. 

2.  Resolving  'power  (correct  definition  being  presupposed)  may  be  said  to 
stand  in  a direct  relation  to  angular  aperture,  and  consequently  to  the  obliquity 
of  the  rays  which  can  be  received  from  the  surface  of  an  object. 

To  measure  the  angular  aperture  of  an  object-glass^  place  the  microscope,  with 
the  body  horizontal,  on  a thin  board  which  turns  on  a pivot  exactly  under  the 
focus  of  the  object-glass  ; set  a lamp  on  a level  with  it  a few  yards  distant;  then 
having  directed  the  body  of  the  microscope  so  far  on  one  side  of  the  light  that 
only  half  of  the  field  is  illuminated,  leaving  half  of  it  dark,  trace  a line  by  the 
edge  of  the  board  on  which  the  microscope  stands ; now  revolve  the  microscope 
horizontally  to  the  other  side  of  the  light  till  only  the  opposite  half  of  the  field  is 
illuminated ; the  angle  now  formed  by  the  edge  of  the  board  and  the  line  pre- 
viously traced,  is  equal  to  the  angular  aperture  of  the  object-glass.  If  the  object- 
glass  has  a very  large  angular  aperture,  the  line  of  demarkation  between  light  and 
darkness  in  the  field,  will  be  indistinct,  and  the  experiment  must  be  performed  in 
a dark  room,  with  a ray  of  sunlight  entering  through  a narrow  perpendicular  slit, 
by  which  means  the  exact  angular  aperture  will  be  more  readily  determined. 
Without  due  precaution,  errors  of  several  degrees  will  be  made  in  estimating  the 
angular  aperture  of  object-glasses.  Adjusting  the  object-glass  for  a covered  ob- 
ject, will  increase  the  angular  aperture  of  some  object-glasses  ten  or  fifteen 
degrees.  Extending  the  draw-tube  has  a similar  effect.  Hence,  in  comparing  the 
angular  aperture  of  two  objectives,  they  are  to  be  examined  with  the  same  eye- 
piece, similar  corrections  for  glass  cover,  and  the  same  length  of  tube. 

Some  methods  employed  to  determine  angular  aperture,  reall-y  determine  noth- 
ing but  the  angular  breadth  of  the  field  of  view,  which  is  often  less  than  the 
angular  aperture  for  an  object  in  the  focus  of  the  microscope.  The  real  question 
to  be  determined  is  the  angular  aperture  of  the  object-glass  as  ordinarily  used  hi 
the  microscope  ; not  what  is  its  angular  aperture  in  other  conditions  where  it  is 
never  placed  for  practical  use. 

3.  Flatness  of  Field.  To  judge  correctly  of  this  quality,  object-glasses  should 
be  tested  with  an  eye-piece  which  gives  a tolerably  large  field  of  view.  In  micro- 
scopes of  inferior  quality,  the  defects  of  the  objectives  are  often  concealed  by  eye- 
pieces so  constructed  as  to  give  but  a very  limited  field  of  view. 

4.  Depth  of  Definition.  The  qualities  already  enumerated,  defining  power,  re- 
solving power,  and  flatness  of  field,  may  all  coexist  in  the  same  object-glass,  but 
there  is  another  quality  so  essential  to  the  prosecution  of  microscopical  researches 
of  a certain  class,  and  which  is  generally  so  little  understood  and  appreciated,  that 
we  shall  dwell  upon  it  more  particularly.  We  refer  to  the  quality  which  some 
object-glasses  possess,  in  addition  to  clear  definition  in  the  focus,  of  giving  tolera- 
ble definition  of  parts  a little  above  or  below  the  exact  focus.  This  quality  may,  per- 
haps, be  called  Depth  of  Definition.,  as  it  refers  to  the  distance  above  or  below 
the  focus  where  definition  ceases,  and  where  objects,  by  their  distance  from  the 
focus,  become  invisible. 

The  value  of  object-glasses  used  for  viewing  tissues  containing  cells  or  vessels 
variously  related  to  surrounding  parts,  and,  in  short,  for  the  practical  every-day 
work  of  the  microscopist,  depends  very  much  on  the  quality  which  we  have  here 
called  depth  or  extent  of  definition. 


CATALOGUE  OF  ACHROMATIC  MICROSCOPES. 

7 


98 


PRACTICAL  DIRECTIONS. 


As  a general  rule,  it  will  be  noticed  that  object-glasses,  of  the  largest  angular 
aperture,  do  not  show  parts  above  or  below  the  focal  plane  as  well  as  glasses  of 
moderate  aperture,  and  yet,  in  this  respect,  a great  difference  is  perceived  in  the 
object-glasses  of  different  makers. 

(a.)  If  two  objectives,  having  the  same  magnifying  power  and  angular  aperture, 
are  both  perfectly  corrected  for  chromatic  and  spherical  aberration,  the  glass 
which  has  the  greater  distance  between  the  anterior  lens  and  the  object,  will  have 
the  greater  depth  of  definition. 

(6.)  If  two  objectives  have  the  same  magnifying  power  and  the  same  distance 
between  the  anterior  lens  and  the  object,  the  glass  having  the  smaller  angular  aper- 
ture will  have  the  greater  depth  of  definition,  provided  the  angular  aperture  is  not 
too  small  to  admit  a tolerable  amount  of  light. 

(c.)  It  is  possible,  however,  that  an  object-glass  of  small  angular  aperture  may 
be  made  with  its  focus  so  near  the  anterior  lens,  that  another  glass  of  larger  aper- 
ture could  be  constructed  with  a focus  so  far  from  the  anterior  lens  that  its  depth  of 
definition  might  exceed  that  of  the  objective  of  smaller  aperture.  Such  a glass  of 
large  aperture  would  be,  in  every  respect  and  for  all  purposes,  greatly  superior  to 
the  other. 

(c?.)  But  if  an  object-glass  of  moderate  angular  aperture,  perfectly  corrected, 
has  its  focus  at  i\\Q  greatest  possible  distance  from  the  anterior  lens,  every  addition 
to  the  angular  aperture,  in  another  similar  object-glass,  necessarily  (in  the  present 
state  of  science)  requires  the  distance  between  the  anterior  lens  and  the  object  to 
be  diminished.  Consequently,  with  the  enlargement  of  the  angular  aperture,  the 
depth  of  definition  is  generally  diminished.  If  opticians  could  procure,  for  the 
manufacture  of  object-glasses,  glass  of  such  refractive  and  dispersive  power  as 
they  would  prefer,  it  would  enable  them  to  improve  very  much  the  depth  of  defi . 
nition  in  object-glasses  of  large  angular  aperture. 

Some  microscopists  have  had  objectives  of  large  angular  aperture  provided  with 
a diaphragm,  to  be  introduced  behind  the  posterior  lens,  when  viewing  objects  re- 
quiring greater  extent  of  definition  of  parts  above  and  below  the  focus.  Some 
advantage  may  be  gained  in  this  manner,  but  object-glasses  so  arranged,  cannot 
have  as  great  depth  of  definition  as  if  originally  constructed  with  the  greater 
distance  between  the  anterior  lens  and  the  object,  which  would  be  possible  with 
the  limited  aperture  to  which  these  glasses  are  thus  temporarily  reduced. 

In  enlarging  the  angular  aperture  of  our  object-glasses,  we  have  always  sought 
to  retain  a reasonable  working  focus,  or  distance,  between  the  anterior  lens  and 
the  object,  and  so  to  combine  depth  of  definition  with  the  other  qualities  already 
enumerated,  as  shall  give  to  our  object-glasses  the  greatest  degree  of  efficiency  for 
the  various  uses  of  the  practical  microscopist. 


PRICE  LIST 


OF 

ACHROMATIC  MICROSCOPES  AAD  MICROSCOPICAL  APPARATUS. 


PRICES  OF  MICROSCOPE  STANDS,* 

Exclusive  of  Objectives,  Apparatus  and  Cases,  except  where  mentioned. 

No.  1.  £ducatioaial  IWlicroscope,  (Sec.  45,)  including 
two  eye-pieces,  (Nos.  1 and  2,)  and  1 inch  and  ^ inch  ob- 
jectives of  second  class,  which,  by  combination  with  the 
two  eye-pieces  give  magnifying  powers  respectively  of  40, 

YO,  180,  and  325  diameters,  packed  in  a mahogany  case,  . $45.00 

The  same  instrument,  with  the  following  additional  apparatus, 

(if  ordered  at  the  same  time,)  viz,  Polariscope,  Camera 
lucida.  Stage-micrometer,  Bull’s-eye  condenser.  Animalcule 
cage.  Stage-forceps,  Hand-forceps,  and  a set  of  dissecting 

instruments, 15.00 

No.  2.  Studeiit’§  Microscope,  (Sec.  46,)  two  eye-pieces, 
movable  diaphragm,  and  1 inch  and  ^ inch  objectives  of 
second  class,  (magnifying  as  above,)  with  mahogany  case,  . 50.00 

Three  dissecting  lenses,  magnifying  5,  10  and  15  diameters  re- 
spectively, mounted  so  as  to  be  inserted  in  the  arm  of  the 
microscope  when  the  body  is  removed,  ....  8.00 

No.  3.  Student’s  Microscope,  (Sec.  47,)  with  the  same 
eye-pieces,  objectives  and  diaphragm  as  the  preceding,  in  a 
mahogany  case.  This  is  the  basis  of  a complete  instrument,  60.00 


* All  the  Microscope  Stands  are  so  constructed  that  additional  apparatus  can  be 
supplied  without  requiring  the  instrument  to  be  sent  for  fitting.  It  is  only  neces- 
sary to  state  the  No.  of  the  instrument  in  the  Catalogue,  and  the  No.  engraved 
upon  it. 


100 


PKICE  LIST. 


No.  4,  A.  §tiideait’s  l^arger  Microscope  §taiac3,  (Sec. 

48,)  with  two  eye-pieces,  (Nos.  1 and  2,)  ...  870.00 

No.  4,  B.  The  same  stand  and  eye-pieces,  with  stage  movable 
in  two  directions  by  rack  and  screw,  instead  of  a lever, 

(Sec.  49,) 75.00 

Either  form  of  stage  above  mentioned,  constructed  so  as  to  re- 
volve around  a steady  centre,  extra,  ....  10.00 

No.  4,  C.  The  same  stand  and  eye-pieces  as  No.  4,  A,  witli 

plain  stage, 55.00 

No.  5,  A.  Studeiirs  liargea*  Micrcscopc  SJaad,  made 
more  portable  than  No.  4,  and  adapted  to  be  used  for  dis- 
secting; Lever  stage  revolving  around  a fixed  centre,  (See 
Sec.  50,)  with  two  eye-pieces,  (Nos.  1 and  2,)  . . . 90.00 

No.  5,  B.  The  same  instrument  as  above,  with  stage  movable 

in  two  directions  by  rack  and  screw,  with  two  eye-pieces,  . 95.00 

No.  5,  C.  The  same  stand  and  eye-pieces,  with  plain  stage,  . 65.00 

No.  6.  Portable  Microscope  Stand,  (Sec.  53,)  with  twm 
eye-pieces  carefully  packed  in  a mahogany  case  of  very 
convenient  dimensions,  with  fittings  for  apparatus,  . . 105.00 

No.  7,  A.  r<arge  Microscope  §fa?id,  (Sec.  54,)  with  eye- 
pieces, Nos.  1 and  2, 125.00 

No.  7,  B.  The  same  stand  and  eye-pieces  as  above,  with  stage 
movable  in  two  directions  by  rack  and  screw,  instead  of 

a lever, 130.00 

No.  8,  A.  Inverted  Microscope  Stands  (Sec.  56,)  Con- 
denser with  rectangular  prism,  two  eye-pieces  and  ma- 

hogany  case, 60.00 

No.  8,  B.  Inverted  Microscope  §tand,  more  complete 
than  the  former,  combining  in  its  construction  all  modern 
improvements.  - • • • - , , 

Prof.  Bailey’s  Indicator  Stage,  (Sec.  52.)  applied  to  No.  4, 

C.,  extra,  .........  40.00 

The  same  applied  to  No.  5,  A,  or  to  No.  7,  A,  instead  of  the 

lever  stage  with  revolving  motion,  .....  15.00 

Extra  Compound  Body  with  rectangular  prism,  Chevalier’s 
plan,  (Sec.  51,)  to  be  substituted  for  the  ordinary  body  in 

microscopes.  Nos.  5 and  6, 18.00 

Short  elbow-tube,  with  rectangular  prism,  (Sec.  51,)  for  the 
same  purpose  as  the  former,  to  fit  into  the  draw-tube  of 

Nos.  4 and  7, 12.00 

The  same  adapted  to  Nos.  1,  2 and  3,  . . . . . 10.00 


PRICE  LIST. 


101 


Cases  for  tlie  Microscopes.  Upright  mahogany  case 
for  No.  7,  with  three  drawers  for  object  slides,  &c.,  and 
fitted  for  the  reception  of  all  accessories,  ....  $18.00 

Upright  mahogany  case  for  Nos.  4 and  5,  with  three  drawers, 

with  fittings  for  all  accessories,  .....  15.00 

Ditto  of  black  walnut,  and  with  one  drawer,  . . . 12,00 


Achromatic  Object-Glasses  for  the  Microscopes. 

First  Class.  (See  Section  58.) 


Object  glasses 

Angular  Aper- 
ture about 

Linear  Magnifying  Power 
(draw-tube  closed) 
with  Eye  pieces. 

Price. 

No.  1. 

No.  2. 

1 No.  3. 

2 inch. 

13  degrees. 

20 

35 

60 

$14.00 

1 “ 

25 

40 

70 

120 

18.00 

1 ^ “ 

i 

60 

90 

150 

260 

25.00 

! ditto  without 

1 adjustmeut. 

50 

a 

20.00 

j ^ inch. 

95-100 

200 

350 

600 

30.00 

! 1 o 

i ® 

125-130 

400 

700 

1100 

40.00 

1 1 u 

: T2 

160 

590 

1000 

1600 

60.00 

1 

Second  Class.  (See  Section  61.) 

2 inch. 

10  degrees. 

$ 8.00 

1 “ 

15 

8.00 

i “ 

40 

Magnifying  Powers 
as  above. 

12.00 

65 

15.00 

i “ 

90 

25.00 

Apparatus  for  IJos.  2 and  3. 

SEE  SECTION 

Movable  Lever  Stage, 10.00 

84.  Three  Dark  Wells  and  Holder,  ....  2.50 

110.  Polariscope,  (two  Nicol’s  prisms,)  ....  14.00 


102 


PEICE  LIST. 


Apparatus  applied  to  Microscopes  Nos.  4,  5 and  6. 

SEE  SECTIOS. 

82.  Achromatic  Condenser, ^27.00 

82.  Brass  work  for  ditto,  alone, 7.50 

79.  Movable  Diaphragm  Plate,  .....  5.00 

84.  Three  Dark  Wells  and  Holder,  ....  3.00 

110.  Polariscope,  (Nicol’s  prisms,)  ....  20.00 

118.  Ditto,  with  Revolving  Selenite  Carrier,  . ...  23.00 

69.  Prof.  J.  L.  Smith’s  Eye-piece  Micrometer  and  Goni- 
ometer, ........  15.00 

83.  Nachet’s  Prism  for  oblique  light,  mounted  with  re- 

volving motion, 12.00 

Apparatus  applied  to  Microscope  No.  7. 


82. 

Achromatic  Condenser,  ..... 

28.00 

82. 

Brass  work  for  ditto,  alone,  ..... 

8.00 

110. 

Polariscope,  (NicoPs  prisms,)  .... 

20-25.00 

118. 

Ditto,  with  revolving  selenite  carrier. 

25-30.00 

84. 

Three  dark  wells  and  holder, 

3.50 

69. 

Prof.  J.  L.  Smith’s  Eye-piece  Micrometer  and  Goni- 

ometer, ........ 

20.00 

83. 

Nachet’s  prism  for  oblique  light,  mounted  with  re- 

volving motion,  ...... 

Apparatus  for  the  Inverted  Microscope. 

12.00 

69. 

Prof.  J.  L.  Smith’s  Eye-piece  Micrometer  and  Goni- 

ometer, ........ 

15.00 

no. 

Polariscope,  (with  two  NicoPs  prisms,)  specially  adapt- 

ed to  the  inverted  microscope,  .... 
Brass  plate  and  spirit  lamp  for  heating  objects  while 

25.00 

under  examination,  ...... 

Apparatus  for  Microscopes  in  General. 

5.00 

63. 

Glass  Stage  Micrometer,  mounted  in  brass,  to 

T Ao  of  ioch, 

4.00 

64. 

Cobweb-Micrometer  Eye- piece,  .... 

30.00 

65. 

Ross’  Eye-piece  Micrometer, 

8.00 

66. 

Jackson’s  Micrometer  with  Eye-piece, 

10.00 

68. 

Dr.  White’s  Micrometer, 

2.00 

VRICE  LIST. 

103 

SEE  SECTION. 

73.  Fraunhofer’s  Stage  Screw  Micrometer,  . 

$40.00 

74. 

Wollaston’s  Camera  lucida,  . 

10.00 

75. 

Nachet’s  Camera  lucida. 

8.00 

76. 

Soemmering’s  Steel  Speculum, 

4.00 

80. 

Bull’s-eye  condenser  on  stand. 

6-7.00 

81. 

Smaller  ditto,  ..... 

5-6.50 

84. 

Lieberkuhn  Speculum,  .... 

3.00 

85. 

Erector,  ...... 

5.00 

80. 

Orthoscopic  Eye-piece,  .... 

10.00 

87. 

Compressor,  ...... 

6.00 

88. 

Animalcule  cage  with  screw. 

4.00 

89. 

Simple  animalcule  cage. 

2.00 

90. 

Stage  Forceps,  ..... 

3.00 

Hand  Forceps  of  brass, 

75 

“ “ of  steel,  .... 

50 

91. 

Frog  Plate, 

5.00 

92. 

Instrument  for  cutting  circles  of  thin  glass. 

8.00 

93. 

Instrument  for  making  cells  of  gold  size. 

or  other 

fluids,  ...... 

3.00 

Extra  Eye-piece,  No.  1,  2 or  3, 

5.00 

111. 

Tourmalines  fitted  to  Eye-piece, 

4.00 

117. 

Barker’s  Selenite  Stage, 

10.00 

WE  ALSO  MAKE  THE  FOLLOWING 

Optical  Instruments  for  various  Scientific  Eesearches. 

Crown  glass  prism  of  any  desired  angle,  mounted  on  a small 


brass  stand,  with  two  movements,  .....  $8.00 

Flint  glass  prism,  mounted  as  above, 10-15.00 

Prism  of  rock  crystal,  do.  ......  15.00 

Prism  of  Iceland  spar,  do.  .....  . 1 5.00 

Two  prisms  on  one  stand,  to  demonstrate  the  theory  of 

Achromatism,  ........  1 5.00 

Hollow  prism  for  fluids,  with  variable  angle,  on  stand,  . 30.00 

Biot’s  prism  for  volatile  liquids,  with  perfectly  piano-parallel 

sides  of  glass, ’ . 15.00 

Ditto,  with  two  compartments,  ......  20.00 

Ditto,  with  three  compartments, 25.00 


104: 


PRICE  LIST. 


Apparatus  for  viewing  Fraunhofer’s  fixed  dark  lines  in  the 
solar  spectrum,  consisting  of  small  achromatic  telescope 
on  stand,  with  flint  glass  prism  and  apparatus  to  mount  the 
same  before  the  objective,  and  a screen  with  flue  slit,  . $35.00 

Apparatus  for  Fraunhofer’s,  Fresnel’s,  Herschell’s  and 

Schwerd’s  experiments  on  diffraction,  ....  60.00 

Wollaston’s  camera  lucida,  mounted  for  draftsmen,  with 

clamp  and  the  necessary  adjustments,  &c.,  from  . . 10-18.00 

Wollaston’s  goniometer,  with  telescope,  graduated  circle  5 
inches  diameter,  the  vernier  gives  the  angles  to  singlfe 

minutes, 40.00 

Same,  graduated  circle  6 inches,  with  telescope  and  improved 

arrangement  for  holding  and  adjusting  the  crystals,  . 60.00 

Photometer,  constructed  by  J.  & W.  G.,  ...  60.00 


This  instrument  was  made  first  for  Prof.  Silliman,  Jr.  It  is  ex- 
tremely sensitive,  and  the  results  of  observations  made  with  its 
aid  are  of  a degree  of  accuracy  and  reliability  which,  we  believe, 
has  never  before  been  obtained  by  a photometer. 


Dichroscopic  lens,  for  discovering  dichroism  in  crystals,  . 6.00 

Pocket  achromatic  simple  microscope,  for  the  use  of  bota- 
nists, mineralogists,  and  others,  magnifying  from  15  to  25 
times  linear,  .........  6-8.00 

Goniometer  for  measuring  the  angles  of  larger  crystals, 

(Gambey’s  construction.)  This  instrument  may  also  be 
used  for  ascertaining  the  refractive  power  of  prisms  and 

crystals, 60.00 

Norremberg’s  polariscope,  . . . . . . 80.00 

Stauroscope,  invented  by  Prof.  V.  Kobell,  of  Munich,  . 25.00 

Biot’s  polariscope,  mounted  on  tripod  and  column,  and  with 

graduated  circles,  ........  40.00 

Soieil’s  apparatus  for  measuring  the  angles  of  the  axes  in  bi- 
axial crystals,  and  the  diameter  of  the  colored  rings,  . 60.00 

Soieil’s  sacharimeter,  .......  90.00 

Apparatus  for  showing  the  transient  polarizing  structure  of 

glass  plates  by  unequal  heating,  .....  2.00 

Apparatus  for  producing  the  polarizing  structure  by  bending,  4.00 

Apparatus  for  producing  the  same  by  compression,  . . 3.00 

Nicol’s  prisms,  according  to  the  size,  from  ....  4-10.00 

WE  ALSO  KEEP  FOR  SALE 

Raspail’s  Simple  Dissecting  Microscopes,  with  three  lenses,  . . $12.00 

Glass  slips  for  mounting  Microscopic  preparations,  per  dozen,  . . 

Square  covers  of  thin  glass,  per  dozen,  ......  .26 

Round  covers  of  thin  glass,  per  dozen,  ......  .36 

Microscopic  specimens  at  various  prices. 


. .S.  -VV  ’ '■ 


i 


■n 

-H 


* ' * '..i 


,*■ 


'.V 


4 


I 


f 


. M 


■m 


